Add link checker CI

[HarHarLinks]

Zola can check our links actually work. To profit from this, we should

• document this requirement in CONTRIBUTING or similar and in PR template.
• add CI. probably we only want it to verify internal links since external links would be too slow to check and outside our control anyway hence could break at any time. this is already in CI just by building zola. however it only applies when using zola's special internal link format:
• convert all internal links currently implemented as external links (https://matrix.org/somewhere instead of @/somewhere.md) to proper internal links
• convert all internal links currently using default format (/somewhere instead of @/somewhere.md) to proper internal links

[spaetz]

How internal links should be done:

For example, linking to a file located at content/pages/about.md would be [my link](@/pages/about.md). You can still link to an anchor directly; [my link](@/pages/about.md#example) will work as expected.

src: https://www.getzola.org/documentation/content/linking/#internal-links

UPDATE: note that the @/dir/index.md needs to point to an actual file, so @/dir would not work, if we really want to show the index.md file within. But I just confirmed that zola build will actually abort building if we try to link to a nonexisting local location.

[HarHarLinks]

Thanks! Also relevant: https://www.getzola.org/documentation/getting-started/cli-usage/#check

[HarHarLinks]

Is there a way to deal with pages that are purely generated? E.g. in #2827 we are left with /category/matrix-org-homeserver.

Also links with parameters, e.g. cachebusting? https://github.com/matrix-org/matrix.org/pull/2830/files#diff-e1b3ebe5dfea6cc0c894833a276db1abc39813b2ab1c27d1cda28f2c157cac09R23

[HarHarLinks]

Another special case: https://github.com/matrix-org/matrix.org/pull/2841/files#diff-d06a9abe866ff567e458f52ab0469f9afa453c73ab9b15a0ac13cf889bf1051eR67