[docutils] Add return types for a few docutils table methods

[adamtheturtle]

No description provided.

[adamtheturtle]

cc @donBarbos @srittau - the mypy_primer diff is relevant as I am working to bump types-docutils in Sphinx.

[donBarbos]

How about using Sequence or Iterable instead of list as suggested by mypy_primer?

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

sphinx (https://github.com/sphinx-doc/sphinx)
- sphinx/directives/patches.py:82:16: error: Call to untyped function "run" in typed context  [no-untyped-call]
+ sphinx/directives/patches.py:64:5: error: Return type "list[Node]" of "run" incompatible with return type "Sequence[table | system_message]" in supertype "CSVTable"  [override]
+ sphinx/directives/patches.py:82:16: error: Incompatible return value type (got "Sequence[table | system_message]", expected "list[Node]")  [return-value]

[donBarbos]

It's ok if this narrowing of return types is also true for methods of child classes.
Then just need to correct these types in sphinx