Add superscript plugin to documentation

Also, remove superscript (and subscript) from list of plugins to port.

Author: elijahgreenstein

docs/index.md

@@ -119,6 +119,12 @@ html_string = md.render("some *Markdown*")
 .. autofunction:: mdit_py_plugins.subscript.sub_plugin
 ```
 
+## Superscript
+
+```{eval-rst}
+.. autofunction:: mdit_py_plugins.superscript.superscript_plugin
+```
+
 ## MyST plugins
 
 `myst_blocks` and `myst_role` plugins are also available, for utilisation by the [MyST renderer](https://myst-parser.readthedocs.io/en/latest/using/syntax.html)
@@ -134,8 +140,6 @@ Use the `mdit_py_plugins` as a guide to write your own, following the [markdown-
 
 There are many other plugins which could easily be ported from the JS versions (and hopefully will):
 
-- [subscript](https://github.com/markdown-it/markdown-it-sub)
-- [superscript](https://github.com/markdown-it/markdown-it-sup)
 - [abbreviation](https://github.com/markdown-it/markdown-it-abbr)
 - [emoji](https://github.com/markdown-it/markdown-it-emoji)
 - [insert](https://github.com/markdown-it/markdown-it-ins)