[check-warnings] ENH: Update PR mode to check corresponding changed files only

[mmcky]

For the PR mode in our check-warnings GitHub action it would be great if we can update it so that it only checks the lectures that have changed in the PR. This reduces confusion about cross-reporting issues that may be in other lectures not being worked on.

Essentially we will need to:

1. fetch which {lectures}.md are changing in the PR
2. For each {lecture}.md file run the check-warnings over the output for those lectures in the _build/html

I see we have the option html-path which define where the _build/html folder is located.