Adding HTTP scaling documentation

[arschles]

This PR moves documentation about the HTTP Addon from the HTTP Addon repo. This PR is incomplete at the moment, but below is my idea for organization:

• HTTP appears in the "Docs" dropdown on the nav bar, and directs to /docs/http
• /docs/http contains a simple intro page as well as all of the documents currently in https://github.com/kedacore/http-add-on/tree/main/docs
• /docs/2.4/concepts/ has "Scaling HTTP Applications" on the sidebar. That goes to an overview page, which contains links to /docs/http

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)
• Add scaler intro doc to "concepts" dropdown
• Move documentation from HTTP Addon wiki to subsection (e.g. /docs/http)
• Version the documentation?

After this PR is merged, most documentation from the docs/ directory should be on the site, and removed from the http addon repo. Similarly, the wiki on the HTTP Addon repository should be removed in favor of this site.

Fixes #548

[arschles]

@tomkerkhove can you let me know what you think about this initial layout?

[tomkerkhove]

I am not seeing the HTTP page, is it possible that you forgot to add it?

[arschles]

@tomkerkhove indeed it is. sorry about that - I'll check this out tomorrow and get it fixed up.

[tomkerkhove]

No rush!

[arschles]

@tomkerkhove I added a content/docs/2.4/http section so a page shows up under the /docs/2.4/http/ path, but I'm kind of confused what types of docs should go under there vs. the concept page at /docs/2.4/concepts/scaling-http/. Is there a best practice for that?

[arschles]

@tomkerkhove there are 2 more TODOs on this PR, but I'm moving this out of draft now. I think it would be good to get a basic layout merged, and then I can add more content in follow-ups. If you agree, I'll pull the remaining TODOs out into issues. WDYT?