Make Mermaid.js limit configurable (#16519)

Make Mermaid.js limit configurable (#16519)

* Make Mermaid.js limit configurable

Add `MERMAID_MAX_SOURCE_CHARACTERS` to `[markup]` settings
to make the maximum size of a mermaid render configurable.

Fix #16513

Signed-off-by: Andrew Thornton <art27@cantab.net>
* fixup! Make Mermaid.js limit configurable

* Update custom/conf/app.example.ini

Co-authored-by: silverwind <me@silverwind.io>
* Update docs/content/doc/advanced/config-cheat-sheet.en-us.md

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>