The "Accepting merge requests" label is being re-named to "Seeking
merge requests" to improve clarity. This updates all references in
the documentation.

- the feature flag working group is consolidating and clarifying the
  existing feature flag documentation. See
https://gitlab.com/gitlab-org/gitlab/-/issues/249129#note_523228764 for
more details. This commit removes the process documentation and
https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/76630
moves it to the handbook.

Now mentions that the gitlab repo also needs to be updated
for the version to be deployed to .com.

All docs link should use /ee not /ce

Generating a changelog entry is now required when a merge request
warrants a CHANGELOG entry.

These may cause confusion and are no longer matching our release
processes.

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

Split the FF docs in 3 separate docs to cover 3 different use-cases.

This is a port of 29759

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

After the 7th it is OK to merge code changes that are merged as long it
is behind a feature flag.

This adds a development guide explaining that we are going to use
feature flags more often, why, what the benefits are, and so on.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/49619 for more
information.

Documentation process at GitLab

Closes #46843

See merge request gitlab-org/gitlab-ce!19902