docs: update autopkgtest PPA result URL format

[rkratky]

Description

• Update the autopkgtest PPA result URL from the old results/autopkgtest-<RELEASE>-<LPUSER>-<PPA>/ format to the new user/<LPUSER>/ppa format

Related issue

• Fixes: docs: resolve-a-migration-issue ppa suggestions #269

Checklist

• Read the contributing guidelines
• Build succeeds (make clean-doc && make html)
• Spellcheck passes (make spelling)
• Inclusive language check passes (make woke)

Additional notes

This PR only addresses the clear URL format change from the issue. It does not address:

• The mention of an unnamed Launchpad group that grants test-request power to non-uploaders (the reporter didn't know the group name)
• The unverified claim that "you can't test autopkgtests in a PPA until after that package has autopkgtests in the Ubuntu archive"

These would need input from the Ubuntu Release Management team.