Add documentation how to render PlantUML in code blocks. (#9894)

* Add documentation how to render plantuml in code blocks.

* Review comments

* Apply suggestions from code review

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>
Co-authored-by: zeripath <art27@cantab.net>

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -18,7 +18,8 @@ menu:
 Customizing Gitea is typically done using the `CustomPath` folder - by default this is
 the `custom` folder from the running directory, but may be different if your build has
 set this differently. This is the central place to override configuration settings,
-templates, etc. You can check the `CustomPath` using `gitea help`. You can override
+templates, etc. You can check the `CustomPath` using `gitea help`. You can also find
+the path on the _Configuration_ tab in the _Site Administration_ page. You can override
 the `CustomPath` by setting either the `GITEA_CUSTOM` environment variable or by
 using the `--custom-path` option on the `gitea` binary. (The option will override the
 environment variable.)
@@ -139,6 +140,39 @@ you would need to remove `{{if .RequireHighlightJS}}` and `{{end}}`.
 
 Mermaid will detect and use tags with `class="language-mermaid"`.
 
+#### Example: PlantUML
+
+You can add [PlantUML](https://plantuml.com/) support to Gitea's markdown by using a PlantUML server.
+The data is encoded and sent to the PlantUML server which generates the picture. There is an online 
+demo server at http://www.plantuml.com/plantuml, but if you (or your users) have sensitive data you 
+can set up your own [PlantUML server](https://plantuml.com/server) instead. To set up PlantUML rendering,
+copy javascript files from https://gitea.com/davidsvantesson/plantuml-code-highlight and put them in your
+`custom/public` folder. Then add the following to `custom/footer.tmpl`:
+
+```html
+{{if .RequireHighlightJS}}
+<script src="https://your-server.com/deflate.js"></script>
+<script src="https://your-server.com/encode.js"></script>
+<script src="https://your-server.com/plantuml_codeblock_parse.js"></script>
+<script>
+<!-- Replace call with address to your plantuml server-->
+parsePlantumlCodeBlocks("http://www.plantuml..com/plantuml")
+</script>
+{{end}}
+```
+
+You can then add blocks like the following to your markdown:
+
+    ```plantuml
+        Alice -> Bob: Authentication Request
+        Bob --> Alice: Authentication Response
+        
+        Alice -> Bob: Another authentication Request
+        Alice <-- Bob: Another authentication Response
+    ```
+
+The script will detect tags with `class="language-plantuml"`, but you can change this by providing a second argument to `parsePlantumlCodeBlocks`.
+
 ## Customizing Gitea mails
 
 The `custom/templates/mail` folder allows changing the body of every mail of Gitea.

docs/content/doc/advanced/external-renderers.en-us.md

@@ -22,6 +22,8 @@ it is just a matter of:
 * add some configuration to your `app.ini` file
 * restart your Gitea instance
 
+This supports rendering of whole files. If you want to render code blocks in markdown you would need to do something with javascript. See some examples on the [Customizing Gitea](../customizing-gitea) page.
+
 ## Installing external binaries
 
 In order to get file rendering through external binaries, their associated packages must be installed.