Fix hyperlinks in documentation

[justindhillon]

Description

There are multiple broken links in this project. Here is what I have fixed:

https://webpack.github.io/docs/hot-module-replacement-with-webpack.html --> https://webpack.js.org/guides/hot-module-replacement/

https://webpack.github.io/docs/code-splitting.html --> https://webpack.js.org/guides/code-splitting/

https://github.com/edx/edx-platform/blob/master/CONTRIBUTING.rst --> https://github.com/openedx/.github/blob/master/CONTRIBUTING.md

Non-testing Checklist

• Consider any documentation your change might need, and which users will be affected by this change.

Post-review

• Squash commits into discrete sets of changes with descriptive commit messages.

Support my work

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[openedx-webhooks]

Thanks for your pull request, but this repo does not accept outside contributions unless they are under contract. If you think this is an error, please contact @nedbat.

[nedbat]

@feanil You might want to update that message....