Fix index errors

[mockersf]

It is still possible to have cases of Error: We can't have a page called index.md in the same folder as an index section...:

• if a folder containing an index.md has a subfolder containing a _index.md
• if a folder containing an index.md has the same name as a folder containing a _index.md but not the same path

To fix both those cases, keep track of sections as their complete path instead of their separate components

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

(Delete or ignore this section for documentation changes)

• Are you doing the PR on the next branch?

[Keats]

if a folder containing an index.md has a subfolder containing a _index.md

Wait does that even work? We should entirely ignore those

[mockersf]

Well it did on Zola 0.16 😄

For example https://github.com/bevyengine/bevy-website/tree/master/content/learn has an index.md file, https://github.com/bevyengine/bevy-website/tree/master/content/learn/book has an _index.md. It's rendered as https://bevyengine.org/learn/ and https://bevyengine.org/learn/book/

[Keats]

Hmm https://github.com/bevyengine/bevy-website/tree/master/content/learn should definitely be _index.md instead of index.md since there are no assets.
That shouldn't change anything for the rendering of the site afaik except changing the template to render.

I'll leave it on for now will remove it for 0.18 (#2130)