Add navigation.indexes feature to mkdocs-material

[PromoFaux]

What does this PR aim to accomplish?:

Discussed internally, effectively saves a click by making linking the index to the main section header rather than having a subheader for overview/index.

I've purposefully left the Docker section as I will be revamping that entire section in another PR

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[rdwebdesign]

What do you think about renaming the initial "Overview" to "About Pi-hole".

Like this:

---

Probably something like:

nav:
-  - Overview: index.md
-  - 'About Pi-hole':
+  - 'About Pi-hole': index.md
     - 'Contact Us': main/contact.md
    - 'Pi-hole Origins': main/origins.md
    - 'On the Web': main/coverage.md

[yubiuser]

I think it is a bit inconsistently used: some item suse the index feature, others not (e.g. About Pihole, Getting Started). I think it should behave the same way everywhere.

[rdwebdesign]

I think it is a bit inconsistently

That's why I suggested the change above.

[PromoFaux]

I think it should behave the same way everywhere.

The difficulty here is inventing content for the index pages of those sections. I think the only ones where this appears inconsistent now are:

• Getting Started
• Docker (Will be addressed in another PR with more Docker docs)
• Contributing

[rdwebdesign]

• Contributing

Every page under "Contributing" is related to code contribution in github.

You can do the same as I suggested above for the initial page.
Remove "Contributing", move the content from "Contributing on GitHub" to the top level and fix the links: