DOC-2381: add new `Math` Plugin documentation for TinyMCE 7.1.

[kemister85]

Ticket: DOC-2381

Site: Staging branch

Changes:

• add new Math plugin documentation for TinyMCE 7.1.

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/
• [x] modules/ROOT/nav.adoc has been updated (if applicable)
• [x] Files has been included where required (if applicable)
• [ ] Files added for New product features, and included a release note entry.

Review:

• [ ] Documentation Team Lead has reviewed