Add strikethrough / deleted node type

[Franklin Koch]

This should be a simple text styling node like strong, underline, etc.

see -
https://github.com/syntax-tree/mdast#delete
https://github.com/syntax-tree/mdast-util-gfm-strikethrough

[Chris Sewell]

Yeh I added it recently in myst-parser: https://myst-parser.readthedocs.io/en/latest/syntax/optional.html#strikethrough
Although it's a bit annoying that docutils doesn't have a "native" node for it