Add Vale .ini for grammar linting and basic tests

This MR is for #38053 (closed) and adds:

• a configuration file for Vale (https://github.com/errata-ai/vale), a grammar / language linter, into the doc directory

• a doc/vale directory to contain Vale styles

• Rudimentary tests, including:

  • Oxford comma
  • Spell-checking
  • Sentence length > 25 words
  • Unclear antecedents (the phrase 'this is')

The MR adheres to our value of MVC and does not attempt to implement Vale at a CI level.

To test out Vale locally with Visual Studio Code, complete the following actions:

• Install Vale locally - brew install vale is easiest
• For local editing in Visual Studio Code, install the Vale extension from https://marketplace.visualstudio.com/items?itemName=testthedocs.vale
• Close and reopen Visual Studio Code.
• Open the doc directory as a new workspace in Visual Studio Code.
• Open the Markdown file you want to lint. The output pane at the bottom of the Visual Studio Code window should display a list of errors, warnings, and suggestions. The list is updated when you re-save the document.

(Other editors are also supported, but I have not tested them.)

cc @marcel.amirault as I know he's working on implementing markdownlint.

Todos before closing this MR:

• Create issue to determine how capitalization tests can be implemented
• Create issue to add documentation around these changes once they've solidified
• Test if it's possible to define the regex for some of our current markdownlint tests to use them as vale tests instead