docs: upgrade mkdocs and fix broken links

[mloiseleur]

Description

It upgrades the whole stack.

Checklist

• Unit tests updated
• End user documentation updated

Additional notes

mkdocs serve is clean, except a Warning on /version.json:

WARNING -  [14:24:16] "GET /versions.json HTTP/1.1" code 404

Hopefully, it will be served by mike, but I haven't found how to launch successfully mike on a local branch.

[mloiseleur]

@szuecs Thanks for your review.

For your other comments, the problem is the way it works currently.
The README.md is used as front page doc on https://github.com/kubernetes-sigs/external-dns
It's also copied in docs/ before generating the mkdocs website.

That makes relative links broken. So I see two solutions:

1. Split this README into two : one in / and the other in docs/ of this repo
2. Try to generate the mkdocs without moving the README.md in docs/

Wdyt of those options ? any other idea ?

[Raffo]

@mloiseleur the second option maybe? I'm not sure aware of the consequences, but duplicating the README is not super great. If we can have an alternative document that doesn't require us to update things in two places every time, then I'm also fine with 1.

[mloiseleur]

@Raffo @szuecs mkdocs does not support to use README.md at the root level, see this upstream issue. Nevertheless, there is a plugin that workaround this.
The plugin seems to work, so I give it a go.

[Raffo]

Thanks! Ping me for a review anytime.

[szuecs]

from my side lgtm

[mloiseleur]

@Raffo @szuecs PR has been rebased. I also added a link to doc website.

[Raffo]

/lgtm
/approve

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