Update the documentation

[hach-que]

The documentation hasn't been updated since 2023 and it's getting a bit crusty. For example, I'm pretty sure uet build --strict-includes doesn't work on UE5 because IWYU mode is broken for launcher engines, and the parameters for --plugin-package changed with the release of Fab.

• Maybe move the UET docs to GitHub Pages instead of being in our GitLab docs repository? Can we put the docs in this repository and then publish to GitHub Pages this way? I might also be able to use a CloudFlare routing/proxy rule to forward just the existing https://docs.redpoint.games/uet/ subpath to point at what GitHub is hosting. Alternatively we could deploy the docs to a CloudFlare worker which will definitely work.
• Check that --strict-includes should still be an option we advertise in the docs.
• Update the documented --plugin-package parameters.
• Actually document the BuildConfig.json sections of the documentation.
• Make sure said sections cover the new uet test parameters for predefined tests.
• Document the uet config options under "Change how Unreal Engine compiles C++ code"
• Update references to ".NET 7" to refer to ".NET 8" (or maybe omit the version number entirely).
• Document uet format.
• Document uet apple-cert.
• Change GitLab links to point to GitHub instead.
• Update Unreal Engine version number everywhere (e.g. "5.2" -> "5.5").
• Make sure the UEFS section gets updated/documented after Replace UEFS daemon with inline mount/copy code #60.