docs: migrate docs hosting site to ubuntu.com/docs

[tang-mm]

This pull request updates the Testflinger documentation configuration and assets to align with its new hosting location at ubuntu.com/docs/testflinger.

Description

Changes following the migration guide at https://github.com/canonical/RTD-Proxy/blob/main/docs/how-to/migrate.rst

• Updated URL and product slug in docs/conf.py to use the new ubuntu.com/docs/testflinger domain,
• Added a templated JavaScript file overwrite_links.js to support dynamically generated links in the ReadTheDocs flyout (version indicator in the bottom-right corner) to point to the new domain.

Resolved issues

CDOC-368

Related proxy configuration in Launchpad:

• Staging: https://code.launchpad.net/~tangmm/rtd-proxy-config/+git/rtd-proxy-config/+merge/504978
• Prod: TODO

Documentation

N/A (refer to the original migration guide )

Web service API changes

NA

Tests

This PR can only be merged when:

• after the staging proxy is deployed, manual validation at stating.ubuntu.com/docs/testflinger
• after the prod proxy is deployed, manual validation at ubuntu.com/docs/testflinger

(After the PR merge, RTD should be configured to use main as default branch)