Fix Github pages relative paths

[sprater]

Fix relative paths for Github pages documents

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What does this Pull Request do?

Modify the relative paths to resources under docs (Github pages) to remove /fits from the beginning of the path, as the fitstool.org domain now points to the fits repo Github pages as its root.

How should this be tested?

1. Clone the repo in an environment that also has a GUI with a browser.
2. Checkout the branch "fix_rel_paths".
3. Change to the docs/ directory.
4. If necessary, run bundle install to install all the jekyll dependencies. If running Ruby 3.4+ also run bundle add bigdecimal.
5. Launch the test webserver: bundle exec jekyll serve
6. In your browser,go to http://127.0.0.1:4000/ (as shown in the output of the bundle exec jekyll serve command)
7. Confirm that the github pages display properly, and that the links work.

Interested parties

@awoods @aurora-charlow @ebenenglish

[awoods]

Was the "User Manual" link missed in https://github.com/fitstool/fits/blob/5f0230e8df599a1f8f287e776c16436d02507c51/docs/quick-start.md:

[awoods]

See general comment on PR

[sprater]

@awoods Good catch, that one escaped my regex. I fixed it, and I made sure there were no more links with that markdown syntax.
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