Fix up bad syntax for documentation (#15939)
Closes #13333
Summary of the issue:
To aid with converting to markdown, fixing txt2tags syntax issues will ensure the conversion works as expected.
Note that not all syntax issues are fixed, however this PR intends to catch easy fixes that can be fixed with regex.
This may result in bad syntax in the markdown files, however these are syntax issues in the current documentation so conversion is unlikely to be a step in a worse direction.
Description of user facing changes
None
Description of development approach
Regex groups:
([^`])`([^`]) replaced with $1``$2
^ ( )*- to detect lists indented by an odd number of spaces
Unmaintained files in user_docs have been removed
