Some markdown files (including root README.md + docs/DOCS.md) contain broken links #352

Hi, happy early new year :)

I wanted to share an issue I was facing. I wanted to look into how to contribute to Tangled, since I'm planning to move some of my projects from GitHub to Tangled. However, I found that a few links when clicking around no longer lead anywhere.

Here is what I've found:

• On the discord for the #development channel, the description contains a link to https://tangled.org/tangled.org/core/blob/master/docs/contributing.md, which doesn't work
• I went to visit the README.md's current docs section instead, which mentions links to "knot hosting guide", "contributing guide", and "hacking on tangled" but don't work
• When visiting https://tangled.org/tangled.org/core/blob/master/docs/DOCS.md, I found the relevant sections they were moved to, but a few links didn't work under the https://tangled.org/tangled.org/core/blob/master/docs/DOCS.md#architecture where I wanted to learn about Spindle. Specifically, the links to sh.tangled.spindle.member, sh.tangled.repo, sh.tangled.pipeline, and the mentioned spindle manifest link at the bottom of the section also does not work (https://tangled.org/docs/spindle/pipeline.md).

I would like to contribute with a PR and fix these links, however I'm not confident I'd know where to correctly update the 4 links mentioned in the last bullet. Also I'd just to like mention just in case, please don't feel rushed :-) I wanted to write out this issue ahead of time so I wouldn't forget.

Relatedly, I think having a separate, follow-up patch looking into adding a link checker in the CI workflow would be helpful. Maybe something like https://lychee.cli.rs/? I haven't tried this tool before, but it's installable via NixOS (https://lychee.cli.rs/guides/getting-started/), popular and maintained: https://github.com/lycheeverse/lychee/