[docs] Refresh for Gambia release

[docs] Refresh for Gambia release

- s/Fuel@OPNFV/OPNFV Fuel/g;
- added README files for ci/scenarios/patches directories;
- refresh & simplify cluster overview diagrams;
- unify labels across docs;
- fix TOC numbering;
- remove local labs PDF/IDF files, as they are merely duplicates of
  Pharos files included as a git submodule;

JIRA: FUEL-397

Change-Id: I87f61938eeb67f13fd9205d5226a30f02e55d267
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
(cherry picked from commit 170d2d1c195d001d6ca786364aaf3c10e714ae36)