Update landing page

Why "HTML" documentation?
I took the intro from docs.plone.org which seems to fit quite well.

Author: ksuess

docs/index.md

@@ -11,7 +11,20 @@ myst:
 
 # Plone 6 Documentation
 
-This website is the HTML documentation of Plone, an open source, enterprise-level content management system.
+This is the community-maintained documentation for the Plone content management system.
+
+This documentation is for:
+
+- Content editors who write, update, and order content on a site
+- Site administrators who install Plone and add-ons, and set up a site
+- Designers who create site themes
+- Deployers who configure servers for site hosting
+- Developers who customize a site’s capabilities, create add-ons, and contribute to Plone itself
+
+```{seealso}
+The documentation for Plone 5 can be found here: https://docs.plone.org/.
+```
+
 
 ```{toctree}
 :maxdepth: 2