docs: fix md redirections for multiversion support

[dgarcia360]

Related issue & explanation scylladb/scylladb#23957

This should allow navigation between versions without being redirected to the main index page:

Note: This fix will only apply to future versions unless backported. While backporting may not be worth the effort, it's important to be aware that the issue will remain in older versions

[Bouncheck]

I have tried multiversionpreview limited to 2 last branches and I cannot get it to work. I still get redirected to main page for a version.
Here's what i did, similarly to as instructed in other PR:

• Made local scylla-4.15.0.x and scylla-4.14.1.x branches and also pushed them to my own remote with applied identical change to source_suffix as in PR.
• Returned to local branch with this PR checked out.
• Edited BRANCHES to contain only those two versions, edited smv_remote_whitelist (on separate runs i tried the values None, "", r'^myrepo$')
• Ran make multiversionpreview in docs subdirectory.

No matter the configuration I'm getting redirected from (for example) http://127.0.0.1:5500/scylla-4.14.1.x/manual/api_conventions/index.html
to http://127.0.0.1:5500/stable/index.html when changing version from dropdown menu (4.15.0.x is "stable" in this case)

[Bouncheck]

Regarding the version list: I see that it's severely outdated. I'll try to sort it out separately. Should be as simple as pushing new .x branches and updating the list.