rc: migrate documentation to Haiku internals pages

docs/develop/ide: A quick guide for haiku code completion

* Works under a Linux "cross-compile" environment
* We can't check in compile_commands.json unforunetly because it
highly depends on your build environment
* An in-tree .clangd is a thing, but our includes are complex
and directory-spefific.
* Thanks to Pulkomandy for adding the flag to jam to do this!

Change-Id: I3be4084c43f7b822bb04ea7ec527c5fbe03d7289
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7158
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>

docs/develop: add some documentation about our Jam usage

Change-Id: I18c1fd3192468a14205bf192b3d1431397d1cc1d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6086
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

docs/develop: explain vendor branches for gcc and buildtools

Change-Id: I9bb3b4efd2d02013da1ec6656adb49cd7d050e86
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5476
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: nephele <nep@packageloss.eu>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Add some docs about our compilers and patches done to them.

Change-Id: I60e0960499aec789ae6240eec949249adb2e35ad
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5224
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>