Add mkdocs.yml JSON schema support for sane_lists and Neoteroi markdown extensions

[AdrianoKF]

As suggested in #6378, this PR adds support for several markdown extensions to the JSON schema for mkdocs.yml:

• sane_lists from python-markdown
• The Cards, Timeline, Gantt, and Spantable markdown extensions from neoteroi-mkdocs (I've added a separate external/ folder under the folder containing the schemas for markdown extensions - does that make sense?)

[squidfunk]

Thanks! Could I please ask you to split the configuration similar to what we did for plugins here?
https://github.com/squidfunk/mkdocs-material/blob/master/docs/schema/plugins.json

The idea is to group external Markdown extensions that are not natively supported by Material for MkDocs under a dedicated group inside the schema. This makes it more clear what we support and what is supported through third parties.

[AdrianoKF]

Sure, that makes perfect sense! Now that I see that the plugins schema already references some external schemas by URL, I might split the Neoteroi part of this PR off and submit it as a PR on their repo instead (and then change this PR accordingly).

As for sane_lists, I would leave it in the current place, since it is part of the official python-markdown extensions (although not explicitly mentioned in the mkdocs-material docs)

[squidfunk]

Sure, that makes perfect sense! Now that I see that the plugins schema already references some external schemas by URL, I might split the Neoteroi part of this PR off and submit it as a PR on their repo instead (and then change this PR accordingly).

That would in fact be perfect and favorable over hosting the external schema here. In fact, we try only to host external schemas for extensions we mention on our docs, so this would be even better!

[squidfunk]

Could you please split this PR into adding sane_lists and the other extension? We can merge sane_lists right away, but we'll wait until the other schema has been added to the upstream repository.

[AdrianoKF]

In fact, the Neoteroi maintainer has merged the PR already. I have updated this PR with the external URL accordingly.

Happy to split this PR, if you'd prefer it for bookkeeping reasons, but otherwise it should be go to go as is. 👍🏻

[squidfunk]

Perfect, that was quick! No need to split into a separate PR – as it's already merged, all good.