Aib improve accessibility

[amaiaita]

Description

❓What: accessibility improvements as follow ups from the accessibility review: https://nhsd-confluence.digital.nhs.uk/pages/viewpage.action?pageId=902212969

🧠Why?: important for our website to be accessible

👨‍💻How?: details of changes made are in the above confluence page

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)? (need to make a new one specific to NHSE Data Science)
• 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

• Github Codespaces - 2-core, 4GB RAM, 32GB hard drive
• devcontainer.json describes further settings