Loop in the next -> next -> next ... chain

[jnweiger]

• Browse https://doc.owncloud.com/server/10.13/admin_manual/installation/

• Check the left sidebar, open the folding triangles, so that all entries under the 'Installation' heading are visible

• The next visible entry below 'Configuration Notes and Tips' is 'Installation Options'.
• Note that this entry has the same URL as the Installation page, that we started with. BAD
• This has the effect, that following the Next links at the bottom of Configuration Notes and Tips loops back to Installation. BAD
• The Heading and text in https://doc.owncloud.com/server/10.13/admin_manual/installation actually discusses Installation Options.

Expected behaviour:

• The chain of Next links is linear.
  • It should neither loop back
  • nor skip any pages.
• https://doc.owncloud.com/server/10.13/admin_manual/installation should be a short general introduction (e.g. mention our built in dependency checker, to convey some confidence in the process), and not discuss Installation Options.

[mmattel]

There is a fix, see referenced PR.

BUT: dont try to intend to fix with such things the currently missing pdf creation. This will come with Antora 3 which provides a clean nav access...