docs: add jekyll landing page

[casperdcl]

Describe your changes

Using jekyll (picked https://phlow.github.io/feeling-responsive which seems to be more maintained than https://mmistakes.github.io/minimal-mistakes)

• content (moved to docs versions + updates #1579)
  • https://ccpi.ac.uk/cil/
  • Tomography-RALTeam/Website content
• fix logos (remove hardcoded /assets/img/)
• fix nav dropdowns prefixing {{site.basurl}} fix navigation dropdown relative links Phlow/feeling-responsive#276
• auto-fix _config.yml:url
• update colours
• update contributing documentation
  • update docs/Makefile
  • how to build locally: make -C docs dirhtml web-deps web serve
  • how to check CI: GHA Art[ie]facts DocumentationHTML unzip into CIL subfolder before running http.server
• GHA DocumentationHTML: use /prnum_merge instead of /nightly
• add subpages
• add more "widgets" moved to docs versions + updates #1579
• fix duplicate refs.bib entries

Describe any testing you have performed

Link relevant issues

• part of docs versions + updates #1579
• follow-up to docs: native build, update sphinx, add versioning #1679

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
• I confirm that the contribution does not violate any intellectual property rights of third parties

[gfardell]

What is the best way for us to review this? Are there build instructions so we can view it, or could you set up a (short) meeting to show us it if you have it running locally?

[casperdcl]

Instructions for testing are included in the docs in this PR.

I ran this:

1. download & extract DocumentationHTML from the CI
2. python -m http.server (for now accessible at http://stfc.cdcl.ml)

[MargaretDuff]

If it is a quick fix, can this paragraph

have slightly bigger text, by default, and be the first thing on the page?

[MargaretDuff]

Looks good to me. Let's get it in so we have a landing page we can then adapt later on

[casperdcl]

If it is a quick fix, can this paragraph have slightly bigger text, by default, and be the first thing on the page?

not a quick fix unfortunately, but added to #1579 :)