docs: Fix 404 links in docs

[MBM1607]

Fixes: #20303

[sonarcloud]

Quality Gate passed

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See analysis details on SonarCloud

[antobinary]

@MBM1607 Thank you for these pull requests!
I am just curious -- how did you detect the broken links? Two weeks ago I went through I built the docs locally and in the process I fixed all the warnings (broken links) -- 5c60542
Maybe there's a different way you used to detect them and I'll be grateful if you share it.

[MBM1607]

@antobinary I didn't actually scan for these links, I was just reading through the docs when I noticed a broken link to https://bigbluebutton.org/, which prompted me to check all the links to the site.

These don't show up as warnings by Docusaurus during the build because those warnings are only for internal documentation links.

---

Now that you mentioned it, I actually went and ran a check using Ahref's Free Broken Link Checker, it reports 8 more broken outbound links which also need to be fixed. There are also inbound links to the docs that are broken, among these the references from the BigBlueButton code comments can at least be fixed.

Let me know if I should go ahead with making a pull request for these updates as well.