editorial fixes in apex docs

- add gitignore entries
- drop common files under docs/etc
- fix documentation titles
- fix doc8 violation (length of lines)
- removal of '_sha1_' for doc tracking
- removal of opnfv logo insertion

Change-Id: Ib0b39704c115f896f62a8ed8ec52257fcf84d544
Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>