ci: ignore repo-local and localhost links in mdox link validation (fixes #2879)

[santhiprakash]

Problem

The mdox link validation step added in #2820 reports false positives for two common link patterns in the docs:

• Repo-local absolute URLs such as /docs/prometheus/latest/configuration/configuration/#kubernetes_sd_config or /docs/visualization/browser. These are website routes that are generated by the prometheus.io site, not files present in this repository.
• Tutorial links to localhost, e.g. http://localhost:9090/graph?..., which are not reachable in CI.

This was reproduced with docs/guides/file-sd.md and docs/guides/cadvisor.md.

Root cause

mdox v0.9.0 only applies configured validators to http(s):// links. Local/absolute links are always validated directly against the filesystem, so an ignore regex like ^/.* in the validator config has no effect. The explicitLocalValidators feature that routes local links through the configured validators is present in a later commit but has not been released yet.

Fix

• Pin the mdox version in the Makefile to commit e88e0a8655a0c91ef6a849edd29798e7e4809d75, which adds explicitLocalValidators and the local validator type.
• Update .mdox.validator.yaml:
  • Set explicitLocalValidators: true.
  • Keep the existing http(s|)://.* ignore rule (this already covers http://localhost and all external links).
  • Add a ^/.* ignore rule for repo-local absolute URLs.
  • Add a catch-all local validator so relative local links (e.g. ./node-exporter.md) are still checked.
• Reformat docs/guides/cadvisor.md with the updated mdox so the example mdox fmt --check invocation passes (docs/guides/file-sd.md was already formatted).

Verification

# The original repro command now passes for both guides:
mdox fmt --links.validate --links.validate.config-file=./.mdox.validator.yaml --check docs/guides/file-sd.md
mdox fmt --links.validate --links.validate.config-file=./.mdox.validator.yaml --check docs/guides/cadvisor.md

# CI-style check is still green and scoped to the OpenMetrics spec:
make check

Risks

• The mdox pin is to an unreleased commit, not a tagged version. This is required because the local-validator feature has not shipped in a release. The Makefile now installs from a stable Git SHA.
• docs/guides/cadvisor.md contains formatting changes produced by the newer mdox (GFM table, list numbering, blank lines around headings). No documentation content was changed.

Fixes #2879

cc @jan--f