Add some mkdocs customizations

[noelmcloughlin]

This PR proposes some default customization for mkdocs

• Theme: Add a documentation icon
• Features: Add some useful extensions for user experience
• Extra javascript: Placeholder file for possible usage in future if mermaid issues arise
• Markdown extensions: Enable code highlighting (superfences) and display diagrams (custom_fences).

Comments or amendments accepted.

Ref:

• feat(docs): upgrade mkdocs/mermaid linkml-project-cookiecutter#100
• https://github.com/nfdi4cat/pid4cat-model/blob/983640826af5f53c6a240f70b30a58a94b88aaf6/mkdocs.yml#L44

[dalito]

Do you plan to add more to this PR or is this ready for review/approval?