Add linting for markdown files
Fixes #16453
Summary of the issue:
Currently, NVDA's markdown files are not linted, and are quite messy and inconsistent as a result.
Description of user facing changes:
None
Description of developer facing changes:
Added a pre-commit hook that lints markdown files.
Description of development approach:
Linted all markdown files that aren't part of submodules or archival copies.
Turned off lint rules that were inappropriate for our repo as I went.
Added the final config to .markdownlint.jsonc (widely supported), and config specific to the pre-commit hook and VS Code plugin to .markdownlint-cli2.jsonc.
