Fix link hover color to be visible (#66)

[alekyapulikota248]

Description

❓What: Fixed the link hover color so it's no longer the same as the background.

🧠Why?: Fixes #66 — links became invisible on hover because the hover color matched the blue background, which could confuse users.

👨‍💻How?: Changed the color value in the .md-typeset a:hover rule in docs/stylesheets/extra.css from the NHS blue variable to #ffeb3b (yellow) so hovered links stay visible against the blue background.

Checklist:

Have checked for the following:

• The website still builds correctly, and you can view it using mkdocs serve.
• There are no new "warnings" from mkdocs
• Does your page follow the page template (or here in Markdown)?
• Spelling errors
• Consistent capitalization
• Consistent numbers
• Material features incorrectly implemented: search for code blocks and markers (e.g. !!!).
• Code snippets don't work
• Images not working
• Links not working

Where it was tested

Edited directly via GitHub's web editor. This was a small, single-line CSS color value change, so it wasn't tested locally with mkdocs serve — happy to verify with a maintainer or add a screenshot if needed.