Docs cleanup fe

See merge request gitlab-org/gitlab-ce!26158

Start moving back to regular fe_docs
Remove Initiatives, add section to index in fe_guide
Delete some no-longer-relevant parts from principles
Update title - Progressive Enchancement is a browser-support term
and could be confused
"When to use" parts are discussed elswhere
Vue & jQuery issue is no longer relevant
Delete duplicate Security docs
Remove empty files, move event_tracking
Shuffle index sections around
Remove sections on Vue & Webpack (we say same thing in prev paragraph)
Remove link to Droplab docs

Adds A short summary of the steps to create and use feature flags
in development for frontend developers who might be unfamiliar
with `rails` and or `flipper`. Adds links to further reading
in the development guide.

Markdown renderers find it easier to determine
where lists start and end when lists are surrounded
by new lines.

For consistency, also ensure entries in the list
are aligned when they span multipls lines.

For the sake of consistency, removes any extraneous
consecutive blank lines from the doc suite.

- Add link to Event tracking in FE guides