[REF] Update README links and simplify Docker Compose instructions

[alyssadai]

• Closes Broken link to environment variable docs in README #339

Changes proposed in this pull request:

• Several outdated links fixed
• Instructions for installing API via Docker Compose mostly removed in favour of a link to our official "Getting Started" docs
• Added GH wf for Linkspector (newer tool w/ more features created by markdown-link-check dev) for checking for broken markdown links in the README.md

Checklist

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• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see our Contributing Guidelines for more info)
• PR has a label for the release changelog or skip-release (to be applied by maintainers only)
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• Tests pass
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• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

[surchs]

Very nice! 🧑‍🍳

Do we have some way of knowing if we have broken links in our README markdown files? Probably not, huh?

[alyssadai]

Do we have some way of knowing if we have broken links in our README markdown files?

good idea, I added markdown-link-check to the pre-commit config! should work the same as in our docs repo.

edit: switched to using new & improved (?) linkspector action by the creator of markdown-link-check, since the latter does not have options to exclude files (i.e.., non-README md files in this case)

[neurobagel-bot]

🚀 PR was released in v0.4.0 🚀