Move example walkthrough from example/ folder into Read the Docs

[harryswift01]

Summary

This PR moves the worked example walkthrough out of the standalone example/ folder and into the Read the Docs site, so it's discoverable by anyone using the published documentation.

Changes

Add a dedicated examples page:

• Added docs/source/examples.rst with the competition calculation walkthrough (command, inputs, outputs)
• Added examples to index.rst's toctree
• Input/output files and the figure are available via Sphinx :download: links rather than embedded inline

Move example assets into the docs tree:

• Moved pbr322.toy.fa, one_line.pbr322.toy.fa, both IRF .html outputs, pbr322.toy.compete.txt, and all.eps into docs/source/_static/examples/
• Removed the now-empty example/ folder and its README.txt

Update cross references:

• Updated source-usage.rst's existing "Example calculation" section to point at the new examples page instead of describing the old directory

Impact

• No behavioural change to SIST itself, this is entirely a documentation move
• The worked example is now visible to anyone using the published Read the Docs site, not just repo clones

[jimboid]

Looks good. I'm not seeing any large diff line numbers so this doesn't suggest the examples are using large files that should be on the file-store, so safe to keep them here.