docs: add submodule setup instructions, renumber BUILD_GUIDE sections

docs: add submodule setup instructions, renumber BUILD_GUIDE sections

- Add new section 2 'Getting the Source' with git clone --recursive
  and fallback 'git submodule update --init --recursive' instructions.
- Note that the build system auto-initializes submodules and applies
  patches if not done manually.
- Fix DOOM section: reference submodule init instead of manual git
  clone; simplify to 'make doom'.
- Renumber sections 3-8 to accommodate the new section.