Fix broken links in docs.airbyte.io

### Topic

We use Docusaurus to build https://docs.airbyte.io — it builds a documenatation website from docs that we have in `./docs` in this repo.

At build time, Docusaurus will check that all _internal_ relative links are correct. But it does not check absolute or extenral links. We have another GitHub Action that checks them: https://github.com/airbytehq/airbyte/blob/master/.github/workflows/doc-link-check.yml — and there are quite a few broken links.

Would be good to get this check back to green.

If you want to contribute all / some of fixes, I would be happy to review them. Sometimes links break just because a page moved, and sometimes it's external vendor documentation, so you have to be careful choosing a new URL.

There are quite a few broken links, so I won't assign this particular issue, but feel free to put together a PR cleaning them up and assign me to be a reviewer. You can find an up to date list of broken links here:

https://github.com/airbytehq/airbyte/actions/workflows/doc-link-check.yml