PMM-15369 - Fix .md→.html link rewrite when link has a fragment

[ondrejpatocka]

Problem

format-release-notes converts relative .md links to absolute
.html links pointing at docs.percona.com. The suffix check ran on
the fully-resolved URL string, so any link with a #fragment after
.md (e.g. QAN-realtime-analytics.md#export-rta-data) no longer
ended in .md and the rewrite was silently skipped, leaving a dead
.md#... link in published release notes.

Confirmed still present in the committed 3.9.0 release notes
(QAN-realtime-analytics.md#export-data), which is why this
previously required manual fixing after every make format-release-notes run.

Fix

Check the .md suffix on the resolved URL's Path component instead
of the serialized string, so the fragment/query no longer interferes
with the check.

[JNKPercona]

Test name	Status
api-tests	passed

[JNKPercona]

Test name	Status
api-tests	passed

[JiriCtvrtka]

Non blocking, because they existed even before:

• cmd/format-release-notes/formatter.go:85 — a bare in-page anchor a is treated as a relative link and rewritten to https://docs.percona.com/percona-monitoring-and-management/3//#section, turning an intra-document jump into a cross-page link to the docs root. extractRelativeURL only checks !IsAbs(); it doesn't exclude fragment-only targets. Since this PR is specifically about fragments, it's a natural add-on: skip when target.Path == "" && target.Fragment != "".
• cmd/format-release-notes/formatter.go:76 — the deliberate 3// double-slash base leaks into output for targets without a ../ prefix (QAN.md#f → 3//QAN.html#f). Cosmetic; docs likely tolerate it.