Fix #104197: Document design principles

Document the design principles. This is basically
https://studio.blender.org/blog/flamenco-3-a-new-beginning/
plus some more info.

web/project-website/content/design-principles/_index.md

@@ -0,0 +1,111 @@
+---
+title: Design Principles
+weight: 25
+---
+
+This page describes some of the design ideas & principles behind Flamenco.
+
+## Target Audience
+
+Flamenco is meant for **smaller animation studios and individuals** at home.
+Think of roughly **1-10 artists** using it, and **1-100 computers** attached to
+the farm to execute tasks. [Blender Studio][studio] uses a handful of servers,
+and combines those with various desktop machines when they're not used by the
+artists.
+
+## Design Principles
+
+The following principles guide the design of Flamenco:
+
+Blender.org Project
+: Flamenco is a true blender.org project. This means that it's Free and Open
+Source, made by the community, lead by Blender HQ. Its development will fall
+under the umbrella of the [Pipline, Assets & IO][PAIO] module.
+
+[PAIO]: https://projects.blender.org/blender/blender/wiki/Module:%20Pipeline,%20Assets%20&%20I/O
+
+Minimal Authentication & Organisation
+: Because Flamenco is aimed at small studios and individuals, it won't offer
+much in terms of user authentication, nor the organisation of users into groups.
+[Custom job types][jobtypes] can be used to attach arbitrary metadata to jobs,
+such as the submitter's name, a project identifier, etc.
+
+[jobtypes]: {{< ref "/usage/job-types" >}}
+
+Minimize External Components
+: Running Flamenco should be extremely simple. This means that it should depend
+on as few external packages as possible. Apart from the Flamenco components
+themselves, all you need to install is [Blender][blender].
+: The downside of this is that development might take longer, as some things
+that an external service could solve need to be implemented. This trade-off of
+developer time for simplicity of use is considered a good thing, though.
+
+[blender]: https://www.blender.org/
+
+No Errors, Guide Users To Success
+: Instead of stopping with a description of what's wrong, like "no database
+configured", Flamenco should show something helpful in which you're guided
+towards a working system.
+
+Customisable
+: Studio pipeline developers / TDs should be able to customise the behaviour of
+Flamenco. They should be able to create new [job types][jobtypes], and adjust
+existing job types to their needs. For this, Flamenco uses JavaScript to convert
+a job definition like "*render this blend file, frames 1-100*" into individual
+tasks for computers to execute.
+
+Work offline
+: Like Blender itself, Flamenco should be able to fully work offline. That is,
+work without internet connection. If any future feature should need such a
+connection, that feature should always be optional, and be disabled by default.
+
+Data Storage
+: Data should be stored as plain files whenever possible. Where a higher level
+of coordination is required, an embedded database can be used; currently
+Flamenco uses [SQLite][sqlite] for this.
+
+[sqlite]: https://pkg.go.dev/modernc.org/sqlite
+
+## Infrastructure & Supported Platforms
+
+Setting up a render farm is not as simple as pushing a button, but Flamenco aims
+to keep things as simple as possible. What you need to run Flamenco is:
+
+- One or more computers to do the work, i.e. running Flamenco Worker.
+- A computer to run the central software, Flamenco Manager. This could be one of
+  the above computers, or a dedicated one.
+- A local network with file sharing already set up, so that the above computers
+  can all reach the same set of files.
+
+Since Blender Studio fully runs on Open Source software, Linux is the main
+platform Flamenco is developed for. Windows and macOS will also be supported,
+but will need help from the community to get tested & developed well.
+
+## Software Design
+
+The Flamenco software follows an **API-first** approach. All the functionality
+of Flamenco Manager is exposed via [the OpenAPI interface][openapi] ([more
+info](openapi-info)). The web interface is no exception; anything you can do
+with the web interface, you can do with any other OpenAPI client.
+
+- The API can be browsed by following the 'API' link in the top-right corner of
+  the Flamenco Manager web interface. That's a link to
+  `http://your.manager.address/api/v3/swagger-ui/`
+- The web interface, Flamenco Worker, and the Blender add-on are all using that
+  same API.
+
+[openapi]: https://projects.blender.org/studio/flamenco/src/branch/main/pkg/api/flamenco-openapi.yaml
+[openapi-info]: https://www.openapis.org/
+
+## New Features
+
+To add a new feature to Flamenco, these steps are recommended:
+
+1. Define which changes to the API are necessary, and update the [flamenco-openapi.yaml][openapi] file for this.
+1. Run `go generate ./pkg/...` to generate the OpenAPI Go code.
+1. Implement any new operations in a minimal way, so that the code compiles (but doesn't do anything else).
+1. Run `make generate` to regenerate all the code (so also the JavaScript and Python client, and Go mocks).
+1. Write unit tests that test the new functionality.
+1. Write the code necessary to make the unit tests pass.
+1. Now that you know how it can work, refactor to clean it up.
+1. Send in a pull request!