Add github action to verify if a redirect was added

[svekars]

This PR adds a GitHub Action that automatically checks if redirects are properly set up when files are deleted or renamed. The action:

1. Runs on pull requests that modify .rst, .py, or .md files in subdirectories excluding files directly in the root.
2. Identifies deleted or renamed files in the PR.
3. Verifies that each deleted/renamed file has a corresponding entry in redirects.py
4. Fails the check if any redirects are missing, prompting the contributor to add them

This gh-action goal is to help maintain proper navigation and prevents broken links when content is moved or removed.

Local testing:

rm -rf beginner_source/text_sentiment_ngrams_tutorial.rst
rm -rf beginner_source/introyt/trainingyt.py
git add -A
git commit -m "Removed test files"
BASE_BRANCH=main CURRENT_BRANCH=$(git branch --show-current) ./.github/scripts/check_redirects.sh

output:

Deleted or renamed files:
beginner_source/introyt/trainingyt.py
beginner_source/text_sentiment_ngrams_tutorial.rst
ERROR: Files were deleted or renamed but redirects.py was not updated. Please update .github/scripts/redirects.py to redirect these files.

Add the following to redirects.py:

redirects = {
    ...
    "beginner/text_sentiment_ngrams_tutorial.html": "../index.html",
    "beginner/introyt/trainingyt.html": "../index.html",
}
git add -A
git commit -m "Updated redirects.py"

Run again:

BASE_BRANCH=main CURRENT_BRANCH=$(git branch --show-current) ./.github/scripts/check_redirects.sh
Deleted or renamed files:
beginner_source/introyt/trainingyt.py
beginner_source/text_sentiment_ngrams_tutorial.rst
All deleted/renamed files have proper redirects. Check passed!

[pytorch-bot]

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[malfet]

Looks fine, though it would be nice to change .sh to .py (at least I'm much more comfortable reading Python code than shell script)