Add SIST logo and refresh README ahead of release

[harryswift01]

Summary

This PR will add the SIST logo to the docs and README and give the README a general pre-release refresh (status badges, citation formatting, contact info) ahead of publishing to Anaconda.

Changes

Add SIST logo:

• Add dark/light SIST logo SVGs under docs/source/_static/logos/
• Wire logos into the Sphinx furo theme via html_theme_options in conf.py
• Embed the logo in the README (dark/light variants via GitHub's #gh-dark-mode-only / #gh-light-mode-only)

Add status badges:

• Add a badges table to the README (Build, Documentation, Anaconda, Quality)

Restructure citations:

• Split the required software citation from the transition-specific papers
• Map each optional paper to its transition type (strand separation, BZ transition, cruciform extrusion) in a table
• Link to CITATION.cff for citing the software itself

Update contact info:

• Replace individual author emails with CCPBIOSIM@stfc.ac.uk

Impact

• No functional/code changes, README and Sphinx docs config only
• Anaconda badges are placeholders that will resolve once the sist package is published to the CCPBioSim channel

[jimboid]

This looks great. We know that the formula works well from other repositories with this structure.