docs: Major updates to the README

[bradenmacdonald]

As discussed on our last call, I have updated the README to make it much more clear what this project is about and who it can help.

[openedx-webhooks]

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[bradenmacdonald]

@felipemontoya or @MoisesGSalas or @jfavellar90 would one of you like to review this?

[openedx-webhooks]

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