Docs: Bunch of small changes in documentation

- Move and rename requirements_docs.txt under /docs folder, preparing for RtD
- Update release process with a note of a branch life-time
- fix typos in 'note' directives

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I370acfbcd7a3d5c2e09ab84702e75b7cb049db35
5 files changed