Merge bitcoin/bitcoin#30034: ci: add markdown link check job

4b7d984269 lint: add markdown hyperlink checker (willcl-ark)

Pull request description:

  Potential followup to: #30025

  This should prevent us reintroducing broken markdown links.

  It does not test "online" (external) links, only those within this repo. Both relative and absolute links are parsed successfully if they resolve.

ACKs for top commit:
  maflcko:
    re-utACK 4b7d984269
  davidgumberg:
    reACK https://github.com/bitcoin/bitcoin/commit/4b7d9842691046b01f0c08d69f924ddb62ccc4c6

Tree-SHA512: 9bc40d700b73499c046bb76157bc139f32ec3850f64ef813bbf7f18f9c01a253abe6a857d6f559890165f2bd26e7742c05d86232cd9b8efb33ff85d735f4f095

ci/lint/04_install.sh

@@ -57,3 +57,8 @@ SHELLCHECK_VERSION=v0.8.0
 curl -sL "https://github.com/koalaman/shellcheck/releases/download/${SHELLCHECK_VERSION}/shellcheck-${SHELLCHECK_VERSION}.linux.x86_64.tar.xz" | \
     tar --xz -xf - --directory /tmp/
 mv "/tmp/shellcheck-${SHELLCHECK_VERSION}/shellcheck" /usr/bin/
+
+MLC_VERSION=v0.16.3
+MLC_BIN=mlc-x86_64-linux
+curl -sL "https://github.com/becheran/mlc/releases/download/${MLC_VERSION}/${MLC_BIN}" -o "/usr/bin/mlc"
+chmod +x /usr/bin/mlc