Add codespell to check for typos

[LukasWallrich]

Description

This adds a spell check to get control of the (not infrequent) typos.

Type of Change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Not yet - waiting for the action to run here, then I will tick the box.

• Successful GH action run

Checklist for Content Editors and Non-Developers

N/A

Checklist for Developers:

• I have attempted to stay aligned to related code in this repository rather than reinventing the wheel.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[github-actions]

👍 All image files/references (if any) are in webp format, in line with our policy.

[LukasWallrich]

Clearly does no work across .md files in content - see #283