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`

Conflicts:
	doc/user/project/slash_commands.md
	doc/workflow/time_tracking.md

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

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

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

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Change location and refactor issue closing pattern documentation

Split into user and administrator docs:

- administration/issue_closing_pattern.md
- user/project/issues/automatic_issue_closing.md

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6466

See merge request !753

Change location and refactor issue closing pattern documentation

## Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

- [x] Make sure the old link is not removed and has its contents replaced with a link to the new location.
- [x] Make sure internal links pointing to the document in question are not broken.
- [x] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
- [x] If working on CE, submit an MR to EE with the changes as well.

See merge request !6466

Split into user and administrator docs:

- administration/issue_closing_pattern.md
- user/project/issues/automatic_issue_closing.md

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Split into user and administrator docs:

- administration/issue_closing_pattern.md
- user/project/issues/automatic_issue_closing.md

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Add docs on Cycle Analytics

Document Cycle Analytics first iteration https://gitlab.com/gitlab-org/gitlab-ce/issues/21170

See merge request !6437

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