Provide table of contents on Explorer artifacts

On extension point pages, there is some kind of "header bar" that plays the role of a table of content, short-linking to documentation and existing contributions:

Sample:
https://explorer.nuxeo.com/nuxeo/site/distribution/Nuxeo%20Platform%20LTS%202019-10.10/viewExtensionPoint/org.nuxeo.ecm.platform.actions.ActionService--actions

This is not consistent with other pages that could also benefit from that kind of header items, typically on Components and Contributions.

Samples:
https://explorer.nuxeo.com/nuxeo/site/distribution/Nuxeo%20Platform%20LTS%202019-10.10/viewComponent/org.nuxeo.ecm.platform.actions.ActionService

https://explorer.nuxeo.com/nuxeo/site/distribution/Nuxeo%20Platform%20LTS%202019-10.10/viewContribution/default-repository-config--configuration

• chose the best rendering for these
• make sure the link is not broken when the section is empty (this is the case on extension points when there are no contributions)