Add documentation portal with MKDocs

docs/docs/installation/gitlab.md

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+## Run a GitLab webhook server
+
+1. From the GitLab workspace or group, create an access token. Enable the "api" scope only.
+2. Generate a random secret for your app, and save it for later. For example, you can use:
+
+```
+WEBHOOK_SECRET=$(python -c "import secrets; print(secrets.token_hex(10))")
+```
+3. Follow the instructions to build the Docker image, setup a secrets file and deploy on your own server from [Method 5](#run-as-a-github-app) steps 4-7.
+4. In the secrets file, fill in the following:
+    - Your OpenAI key.
+    - In the [gitlab] section, fill in personal_access_token and shared_secret. The access token can be a personal access token, or a group or project access token.
+    - Set deployment_type to 'gitlab' in [configuration.toml](./pr_agent/settings/configuration.toml)
+5. Create a webhook in GitLab. Set the URL to the URL of your app's server. Set the secret token to the generated secret from step 2.
+In the "Trigger" section, check the ‘comments’ and ‘merge request events’ boxes.
+6. Test your installation by opening a merge request or commenting or a merge request using one of CodiumAI's commands.