- Edit pages under user/project/merge_requests and
  add the relevant types in the frontmatter.
- Clean up descriptions.

Based on https://gitlab.com/groups/gitlab-org/-/epics/1280

- Remove image for adding new permission scheme
- Delete jira_add_permission_scheme.png as no longer included in content
- add group language
- Reword the add user to group instructions
- Reword the add selected users instruction
- Reword add permission scheme instruction
- Update jira_server_configuration intro

Update screecaps to reflect jira version ~8.1 and update instructions
to include Permission Schemes.

Signed-off-by: NWilliam Desportes <williamdes@wdes.fr>

Introduce default_git_depth in project's CI/CD settings and set it to
50. Use it if there is no GIT_DEPTH variable specified. Apply this
default only to newly created projects and keep it nil for old ones
in order to not break pipelines that rely on non-shallow clones.

default_git_depth can be updated from CI/CD Settings in the UI, must be
either nil or integer between 0 and 1000 (incl).

Inherit default_git_depth from the origin project when forking projects.

MR pipelines are run on a MR ref (refs/merge-requests/:iid/merge) and it
contains unique commit (i.e. merge commit) which doesn't exist in the
other branch/tags refs. We need to add it cause otherwise it may break
pipelines for old projects that have already enabled Pipelines for merge
results and have git depth 0.

Document new default_git_depth project CI/CD setting

Add a link to the gemnasium-db and explain
how to search for a vulnerability or contribute a new one.

See https://gitlab.com/gitlab-org/gitlab-ee/issues/11169

`doc/user/admin_area/img/index_runners_search_or_filter.png` does not
need x permissions.