Move Mattermost doc from Omnibus to main project

doc/administration/index.md

@@ -111,7 +111,7 @@ Learn how to install, configure, update, and maintain your GitLab instance.
 
 ### GitLab platform integrations
 
-- [Mattermost](https://docs.gitlab.com/omnibus/gitlab-mattermost/): Integrate with [Mattermost](https://mattermost.com), an open source, private cloud workplace for web messaging.
+- [Mattermost](../integration/mattermost/index.md): Integrate with [Mattermost](https://mattermost.com), an open source, private cloud workplace for web messaging.
 - [PlantUML](integration/plantuml.md): Create diagrams in AsciiDoc and Markdown documents
   created in snippets, wikis, and repositories.
 - [Web terminals](integration/terminal.md): Provide terminal access to your applications deployed to Kubernetes from GitLab CI/CD [environments](../ci/environments/index.md#web-terminals).

doc/development/architecture.md

@@ -610,7 +610,7 @@ to make sure we were logging responsibly. This is just a packaged version of the
 
 - [Project page](https://github.com/mattermost/mattermost-server/blob/master/README.md)
 - Configuration:
-  - [Omnibus](https://docs.gitlab.com/omnibus/gitlab-mattermost/)
+  - [Omnibus](../integration/mattermost/index.md)
   - [Charts](https://docs.mattermost.com/install/install-mmte-helm-gitlab-helm.html)
 - Layer: Core Service (Processor)
 - GitLab.com: [Mattermost](../user/project/integrations/mattermost.md)

doc/integration/mattermost/gitlab-mattermost.msc

+msc {
+  # Use https://mscgen.js.org or mscgen to convert this into PNG
+  hscale="1.5",
+  wordwraparcs=on;
+
+  user [ label="User", textbgcolor="blue", textcolor="white" ],
+  mattermost [ label="Mattermost", textbgcolor="red", textcolor="white"],
+  gitlab [ label="GitLab", textbgcolor="indigo", textcolor="white"];
+
+  user=>mattermost [label="GET https://mm.domain.com"];
+  mattermost note gitlab [label="Obtain access code", textcolor="green"];
+  mattermost=>gitlab [label="GET https://gitlab.domain.com/oauth/authorize", textcolor="indigo"];
+  gitlab rbox user [label="GitLab user logs in (if necessary)"];
+  gitlab rbox gitlab [label="GitLab verifies client_id matches an OAuth application"];
+  gitlab=>user [label="GitLab asks user to authorize Mattermost OAuth app"];
+  user=>gitlab [label="User clicks 'Allow'"];
+  gitlab rbox gitlab [label="GitLab verifies redirect_uri matches list of valid URLs"];
+  gitlab=>user [label="302 Redirect: https://mm.domain.com/signup/gitlab/complete"];
+  user=>mattermost [label="GET https://mm.domain.com/signup/gitlab/complete", textcolor="red"];
+  mattermost note gitlab [label="Exchange access code for access token", textcolor="green"];
+  mattermost=>gitlab [label="POST http://gitlab.domain.com/oauth/token", textcolor="indigo"];
+  gitlab=>gitlab [label="Doorkeeper::TokensController#create"];
+  gitlab=>mattermost [label="Access token", textcolor="red"];
+  mattermost note gitlab [label="Mattermost looks up GitLab user", textcolor="green"];
+  mattermost=>gitlab [label="GET https://gitlab.domain.com/api/v4/user", textcolor="indigo"];
+  gitlab=>mattermost [label="User details", textcolor="red"];
+  mattermost=>user [label="Mattermost/GitLab user ready"];
+}

doc/user/project/integrations/mattermost_slash_commands.md

@@ -22,7 +22,7 @@ on your configuration:
 
 - **Omnibus GitLab installations**: Mattermost is bundled with
   [Omnibus GitLab](https://docs.gitlab.com/omnibus/). To configure Mattermost for Omnibus GitLab, read the
-  [Omnibus GitLab Mattermost documentation](https://docs.gitlab.com/omnibus/gitlab-mattermost/).
+  [Omnibus GitLab Mattermost documentation](../../../integration/mattermost/index.md).
 - **If Mattermost is installed on the same server as GitLab**, use the
   [automated configuration](#automated-configuration).
 - **For all other installations**, use the [manual configuration](#manual-configuration).