feat: Add deprecated content process documentation (#1401)

This commit adds a process document for reviewing old, deprecated
content. This was always possible thanks to git revision history, but
there was no single reference point available for when deprecated
content was removed.

This new process document is a quick reference point, allowing someone
to check out the relevant state of documentation for a given product
without needing to undergo any git archaeology.

Author: ADubhlaoich

documentation/README.md

@@ -18,6 +18,7 @@ If you're interested in contributing to the [NGINX documentation website](https:
 - [Managing content with Hugo](/documentation/hugo-content.md)
 - [Using include files](/documentation/include-files.md)
 - [Contributing closed content](/documentation/closed-contributions.md)
+- [Review deprecated content](/documentation/deprecated-content.md)
 
 ## Writing guidance
 

documentation/deprecated-content.md

@@ -0,0 +1,57 @@
+# Review deprecated content
+
+As part of the documentation lifecycle, the documentation team periodically removes deprecated content.
+
+Although a product may no longer be available for sale, we maintain the documentation until end of support for the benefit of its users.
+
+By removing the content, their files do not create unnecessary noise when working on documentation, nor impact the user search experience.
+
+Since we use git to track file changes, the documentation is still available by checking out an older revision of the repository.
+
+To quickly traverse git history, we have created tags corresponding to the last commit prior to the removal of the products' files.
+
+## Check out deprecated product documentation
+
+To check out deprecated product documentation, use the `git` command:
+
+```shell
+git checkout <product-tag>
+```
+
+You should replace `<product-tag>` with a tag from the following table:
+
+| Product name                           | Tag                  | Date of removal |
+| -------------------------------------- | -------------------- | --------------- |
+| NGINX App Protect WAF[^1]              | `archive-nap`        | 2025-11-05      |
+| NGINX Controller                       | `archive-controller` | 2025-10-08      |
+| NGINX Management Suite[^2]             | `archive-nms`        | 2025-10-08      |
+| NGINX Service Mesh                     | `archive-mesh`       | 2025-11-05      |
+
+## Review and add tags
+
+You can review the repository tags using the `git tag` command:
+
+```text
+➜ git tag -l
+archive-controller
+archive-mesh
+archive-nap
+archive-nms
+```
+
+To add a new tag, use the following command:
+
+```shell
+git tag -a <tag-name> <commit-sha>
+```
+
+The tag name should follow the format of _archive-\<product-name\>_, and the commit sha should be the last commit **before** the content was removed.
+
+To add the new tag to the remote repository, you must add it as an explicit argument to `git push`:
+
+```shell
+git push origin <tag-name>
+```
+
+[^1]: NGINX App Protect WAF is now known as F5 WAF for NGINX  
+[^2]: NGINX Management Suite was refactored into NGINX Instance Manager