Fix 404 error when docs version switching

[adam-narozniak]

Issue

When switching from main (generally later - a branch that has a new documentation style = a single doc per function/class/module) to an older style documentation the error - 404 page not found - appears.

Description

e.g. from: https://flower.dev/docs/framework/main/en/ref-api/flwr.driver.html#module-flwr.driver
when the version gets switched to 1.6 - then such a page does not exist

Proposal

Fall back to the reference section of the index.html when the page cannot be found.

Explanation

Employ a dynamic check:
If the page exists (in the requested new version),
change to it;
elseif the page does not exist
fall back to the reference section in the idex.html.

A javascript function is created to handle the check. (not the local testing might be harder because the newUrl prefix is fixed as https://flower.dev/docs/framework/ (see function in versioning).

Note that coming back from the old version, to the new version
e.g. from https://flower.dev/docs/framework/v1.5.0/en/ref-api-flwr.html#module-flwr.server
to main (currently only the jump to main can be tested since that's the only version with the new type of docs), currently works by redirecting to the https://flower.dev/docs/framework/main/en/ref-api/flwr.html (which is the entry point of the documentation). I don't know why that mechanism works.

[charlesbvll]

Looks good to me!

[tanertopal]

@danieljanes, I am okay with the solution to this issue. Wdyt?

[danieljanes]

LGTM!