Switch from mkdocs-material to zensical

[yubiuser]

What does this PR aim to accomplish?:

mkdocs-material is in maintenance mode and their creators build a new site generation tool called zensical (here). It offers a back-wards compatibility to mkdocs-material so the transition is quite smooth.

This is work in progress

Internal

• Replace markdown_include with snippets as the former did not work well with zensical (adapts Add deprecation notice to OpenVPN pages [v6] #1029)
• Switch to native zensical.toml
• Decide which theme should be used (classic vs modern)
• Port navigation
• Update Readme.md
• Table of contents > On this page in right navigation panel
• Check snippts (OpenVPN deprecation notice) working
• Tooltip Footnotes
• Abbreviations work when defined in same page, but glossary does not; Tracking

External

• Check if git-revision-date-localized-plugin is still needed, Tracker
• Figure out what replaces theme/features: search.suggest and search.share
• Build mode --strict is missing, affecting link validation
• Check how to implement not_in_nav, Tracker
• Replace mkdocs-redirects, Tracker
• Page headings are not deducted from navigation name but from document title, but they should, Tracking

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[rdwebdesign]

Figure out what replaces theme/features: search.suggest and search.share

I'm not sure if these options are available on the new system, but do we need them?

search.share only adds an icon to the search bar that copies a link to clipboard. The link will include the query string ?q=<search_string>, link this: https://docs.pi-hole.net/?q=dns.

search.suggest is useful (it works like auto-completion on the search field), but I'm not sure if this is available.

The new search window is a little different, but it also shows all results in real time while you type the search string, like mkdocs search. Not sure if the suggestions are needed.

[yubiuser]

The whole search was completely re-implemented. We probably don't need search.share (who is linking to search results which can change on every page change?); search.suggest is nice and might be implemented as they work on the search functionality. I don't think it is a big loss for both option, but just kept track of things that we use and which are currently not implemented.

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Note: I removed markdown_extensions which were unused without listing them above.

[rdwebdesign]

Check how to implement not_in_nav

Not sure if we really need this.

Decide which theme should be used (classic vs modern)

Can you provide a screenshot using the "classic" theme?

[yubiuser]

Not sure if we really need this.

It depends if they implement warning about files not included in the navigation. Tracking here: zensical/zensical#124

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Can you provide a screenshot using the "classic" theme?

This is 1:1 the current Materials for MkDocs theme we use. So all screenshots are at https://docs.pi-hole.net/
(This is why I used the 'modern' them for the Nelify preview so we can easily compare)

[mwoolweaver]

(This is why I used the 'modern' them for the Nelify preview so we can easily compare)

i vote modern. (though i'm sure my vote doesnt count much😂😂)