Closes #23460

Adds `#build_notification_recipients` to `NotificationRecipientService`
that returns the `NotificationRecipient` objects in order to be able to
access the new attribute `reason`.

This new attribute is used in the different notifier methods in order to
add the reason as a header: `X-GitLab-NotificationReason`.

Only the reason with the most priority gets sent.

Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/142

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

delete old doc, update links to the current doc

- correct the file tree
- remove duplicate doc
- move to project/members/
- move images
- update links

fix the file tree

Update gitlab_flow.md, Teatro seems to be completely dead, see also https://forum.gitlab.com/t/gitlab-flow-documentation-teatro/7774

other than just removing a key, which doesn't affect the verified state
of a commit, revoking a key unverifies all signed commits.

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/27070

Deprecate "chat commands" in favor of "slash commands"

We looked for things like:

 - `slash commmand`
 - `slash_command`
 - `slash-command`
 - `SlashCommand`

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

Notification settings are described, but aren't mentioned where they
are or where to change them.

- Move to dicsussions/index.md
- Bring back previous path with a link to new location
- Reorder sections