Add overview of nginx-org source ingestion

Author: nginx-jack

documentation/nginx-org-source.md

@@ -0,0 +1,45 @@
+# nginx.org content
+
+This document describes the process by which https://github.com/nginx/nginx.org are ingested by the `nginx/documentation` repository, and rendered as markdown.
+
+## Source of truth
+The https://github.com/nginx/nginx.org repository remains the source of truth for a subset of content in `/content/nginx`. Primarily, this includes is the `module reference`, and the https://github.com/nginx/nginx.org/blob/main/yaml/nginx_api.yaml.
+
+Any modifications to these autogenerated files will be blocked during the pull request process.
+If modifications are required, they will be made in the source xml in https://github.com/nginx/nginx.org repository.
+
+All files that are autogenerated from this source, will have the warning:
+
+```md
+********************************************************************************
+🛑 WARNING: AUTOGENERATED FILE - DO NOT EDIT 🛑 This Markdown file was
+automatically generated from the source XML documentation. Any manual
+changes made directly to this file will be overwritten. To request or
+suggest changes, please edit the source XML files instead.
+https://github.com/nginx/nginx.org/tree/main/xml/en
+********************************************************************************
+```
+
+## Markdown generation flow
+
+A GitHub Action running as a cron job in the `nginx/documentation` repository detects changes in the documentation within `nginx/nginx.org`, generates new markdown files when changes are found, and creates a PR with these changes.
+
+Only PRs generated by this GitHub Action can be merged.
+
+The markdown files are generated using a markdown-specific XSLT file, which targets only the XML sources intended for inclusion in the nginx/documentation repository.
+
+```mermaid
+flowchart TD
+    Start([Cron Job Triggered]) --> CheckChanges{Changes detected in 'en' docs of nginx/nginx.org?}
+    CheckChanges -- Yes --> CloneRepo[Clone nginx/nginx.org repository]
+    CloneRepo --> RunMake["Execute make target: 'make hugo-md'"]
+    RunMake --> MoveFiles["Move generated markdown files to '/content/nginx' directory"]
+    MoveFiles --> OpenPR["Create PR in nginx/documentation repository"]
+    OpenPR --> Finish([End])
+
+    CheckChanges -- No --> FinishNoAction([End - No updates detected])
+
+    style Finish stroke:#27ae60,stroke-width:2px
+    style OpenPR stroke:#2980b9,stroke-width:2px
+    style CheckChanges stroke:#7f8c8d,stroke-width:2px
+```