Improve learning paths docs with illustration, more cross links, minor corrections

[PhilReedData]

Summary of changes

• Following user feedback, I have added an illustration on the components and relationships within learning paths, topics and materials.
• Image created in Draw.io, exports to SVG, easy to edit online for free.
• Minor corrections to the text on the learning paths about page.
• Links from the side columns on the create new learning path/topic pages, links to the docs.

Motivation and context

• As discussed at TeSS project meeting 2024-12-09.

Screenshots

Checklist

• [x ] I have read and followed the CONTRIBUTING guide.
• [x ] I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.