* Add mermaid JS renderer

For feature parity with GitLab. Tested in files, issues, wiki, editor.
arc-green only does an inversion because the renderer seems to like to
render white backgrounds on boxes.

Ref: https://github.com/go-gitea/gitea/issues/3340
Fixes: https://github.com/go-gitea/gitea/issues/12307

* add feature entry, switch to neutral theme, remove border

* add bindFunctions support

* remove unnecessary border-radius

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>