Presumably the `renderIfEE` is from an older edit.

This is to mainly allow for images to be uploaded in design management.

We also enable the rubocop that makes it mandatory

Signed-off-by: Rémy Coutable <remy@rymai.me>

The new version requires `python3` to be available. In omnibus
installation this is already a reality as we are currently pathing
previous `gitlab-markup` version to use `python3` instead of `python2`.

We are now requiring `python3` with the gem without having to patch it.

As a consequence to also make it easy to use it in development, we've
introduced a `Pipfile` and `Pipfile.lock`, working similarly to
`Gemfile` and `Gemfile.lock`, and added documentation on how to use
them.

The new version requires `python3` to be available. In omnibus
installation this is already a reality as we are currently pathing
previous `gitlab-markup` version to use `python3` instead of `python2`.

We are now requiring `python3` with the gem without having to patch it.

As a consequence to also make it easy to use it in development, we've
introduced a `Pipfile` and `Pipfile.lock`, working similarly to
`Gemfile` and `Gemfile.lock`, and added documentation on how to use
them.

Signed-off-by: Rémy Coutable <remy@rymai.me>

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.

(cherry picked from commit 26cba3b05fbf02e97bb6504382654209af4f8e78)

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57298