Split plugin index page summary for better anchor links

[pedro-psb]

Split the summary into sections, like:

# Welcome to plugin

{{ Plugin index page }}

---

## Summary

### General

* RESTAPI
* Changelog
* Github (maybe add this too)

### User

* User Home  # use plugin/user/index.md if it exists
* Guides
* Learn
* etc

### Admin

# omit Admin Home, if {plugin}/admin/index.md doesnt exit
* Guides
* Learn
* etc

The benefits of this are:

• We can link to the whole "Admin" section of pulpcore, for example, if that doesnt have an index page.
• Some repositories have long ToC (example, pulp-operator). This can help readabillity