docs/develop: add sourcetree layout document

Largely based on a_brief_introduction_to_our_source_repository_layout.html
from the website, revised and updated. I will delete the one from the
website and put a redirect to here once this is merged.

Change-Id: I06657bbea4c16df7bff1f2357cffd2ec8a63c65a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7236
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

docs: document the steps to update GCC buildtools

Change-Id: Iaa41194e0f76594efd504b65f3565b22743dce34

Improve "sourcecode" page of docs/develop/ a bit

Prepare for adding section about getting the sources and submitting
patches (to be converted from the existing docs on the website)

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>