Tinymce customization (#1807)

* Create template for TinyMCE docs

* Migrate content from #1806

* Fix redirecting link to TinyMCE latest

* Update docs/classic-ui/tinymce-customization.md

Co-authored-by: Peter Mathis <petschki@users.noreply.github.com>

* Enhance tinymce customization documentation

* Explain each of the three options to better inform the reader's decision
Restructure the .css file option to be more how-to guide-ish

---------

Co-authored-by: Peter Mathis <petschki@users.noreply.github.com>
Co-authored-by: Peter Mathis <peter.mathis@kombinat.at>

docs/classic-ui/index.md

@@ -34,6 +34,7 @@ static-resources
 template-global-variables
 templates
 theming/index
+tinymce-customization
 viewlets
 views
 whatsnew

docs/classic-ui/tinymce-customization.md

@@ -0,0 +1,87 @@
+---
+myst:
+  html_meta:
+    "description": "TinyMCE customization in Plone 6"
+    "property=og:description": "TinyMCE customization in Plone 6"
+    "property=og:title": "TinyMCE customization in Plone 6"
+    "keywords": "Plone 6, Classic UI, Bootstrap 5, TinyMCE, customization"
+---
+
+(classic-ui-tinymce-customization-label)=
+
+# TinyMCE customization
+
+This chapter is a developer reference manual for customizing {term}`TinyMCE`.
+
+
+## Customize {menuselection}`Format` menu
+
+There are several options to customize TinyMCE's {menuselection}`Format --> Formats` menu.
+
+The first two options use TinyMCE's [`formats` JSON configuration](https://www.tiny.cloud/docs/tinymce/latest/content-formatting/).
+They are less complicated to implement than the third option.
+
+
+### Add-on GenericSetup configuration file
+
+This option is best for system administrators and developers who write their own add-ons to ease reproducibility.
+
+You can add a GenericSetup configuration file to your add-on, such as {file}`profiles/default/registry/tinymce.xml`, with the following content.
+
+```xml
+<registry>
+  <records interface="plone.base.interfaces.controlpanel.ITinyMCESchema"
+            prefix="plone">
+    <value key="formats"
+           purge="true"
+        >{
+        "myformat": {
+            "block": "div",
+            "classes": "my-css-classes"
+        }
+    }</value>
+  </records>
+</registry>
+```
+
+
+### Edit the `Formats` option in the TinyMCE control panel
+
+This option is good for quick edits, but is not as reproducible as the GenericSetup option.
+
+You can manually customize the `Formats` value through-the-web in the TinyMCE control panel.
+
+1.  Navigate to {menuselection}`Site Setup --> TinyMCE --> Default`, or append `@@tinymce-controlpanel` to the root of your website in your browser's location bar.
+1.  Scroll down to {guilabel}`Formats`, and edit the JSON configuration.
+1.  Click the {guilabel}`Save` button.
+
+
+
+### Inject formats with files named {file}`tinymce-formats.css`
+
+This option is more complex to implement than the previous options.
+
+In Plone 6, TinyMCE has a special logic that automatically reads registered files named {file}`tinymce-formats.css` and adds the CSS classes defined in those files to TinyMCE's {menuselection}`Format --> Formats` menu by using the [`importcss_file_filter` option](https://www.tiny.cloud/docs/tinymce/latest/importcss/#importcss_file_filter).
+
+To add custom formats, you can provide your own files.
+
+1.  Name the file {file}`tinymce-formats.css`.
+1.  Register the file as a resource in your Plone site.
+1.  Include the file in the TinyMCE control panel in the textarea input {guilabel}`Choose the CSS used in WYSIWYG Editor Area`.
+    1.  Navigate to {menuselection}`Site Setup --> TinyMCE --> Default`, or append `@@tinymce-controlpanel` to the root of your website in your browser's location bar.
+    1.  Scroll down to {guilabel}`Choose the CSS used in WYSIWYG Editor Area`.
+
+CSS styles defined in this file will automatically be added to the top level of TinyMCE's {menuselection}`Format --> Formats` menu.
+
+
+## Remove imported formats
+
+Plone 6 Classic UI ships with the Barceloneta theme which includes two custom formats, `highlight-inline` and `p.highlight-paragraph`, in the TinyMCE {menuselection}`Format --> Formats` menu.
+You can remove these formats through the TinyMCE control panel.
+
+1.  Navigate to {menuselection}`Site Setup --> TinyMCE --> Default`, or append `@@tinymce-controlpanel` to the root of your website in your browser's location bar.
+1.  Scroll down to {guilabel}`Choose the CSS used in WYSIWYG Editor Area`.
+1.  Remove the entry `++theme++barceloneta/tinymce/tinymce-formats.css`.
+1.  Click the {guilabel}`Save` button.
+
+Once removed, the custom formats will no longer appear in the menu.