chore: enable Mermaid.js diagrams

[HonkingGoose]

Changes:

• Enable Material for MkDocs's built-in Mermaid.js diagram support ¹
• Removing the "search highlights" from the links to the Material to MkDocs manual.

Context:

The new config allows Material for MkDocs to show the Mermaid.js diagrams properly.

The Renovate docs for the Rubygem datasource uses a Mermaid.js diagram.

Footnotes

1. Material for MkDocs, diagrams ↩