Add link validation script

[ras0219-msft]

This PR adds a script to check all documentation-internal links, ensuring that pages and anchors exist. Unfortunately, it operates on the generated html files, so it isn't easy to add as a pipeline step in microsoft/vcpkg for docs PRs.

It already has found several existing issues (which prevent me from adding it as a mandatory pipeline step):

$ node scripts/validateLinks.js
Broken internal link /en/docs/README.html -> /en/docs/specifications/export-command.html
Broken internal link /en/docs/README.html -> /en/docs/specifications/feature-packages.html
Broken fragment link /en/docs/about/faq.html -> /en/docs/users/integration.html#export
Broken fragment link /en/docs/users/config-environment.html -> /en/docs/users/binarycaching.html#Configuration
Broken fragment link /en/docs/users/config-environment.html -> /en/docs/users/binarycaching.html#Configuration
Broken fragment link /en/docs/users/config-environment.html -> /en/docs/users/binarycaching.html#Configuration
Broken fragment link /en/docs/users/config-environment.html -> /en/docs/users/binarycaching.html#NuGets-cache
Broken internal link /en/docs/users/integration.html -> /en/README.html
Broken fragment link /en/docs/users/integration.html -> /en/docs/users/triplets.html#VCPKG_CHAINLOAD_TOOLCHAIN_FILE
Broken fragment link /en/docs/users/manifests.html -> /en/docs/users/integration.html#cmake
Broken fragment link /en/docs/users/manifests.html -> /en/docs/users/integration.html#with-msbuild
Broken fragment link /en/docs/users/triplets.html -> /en/docs/users/integration.html#triplet-selection
Broken internal link /en/docs/maintainers/portfile-functions.html -> /en/docs/maintainers/ports/vcpkg-gn/vcpkg_gn_configure.html
Broken internal link /en/docs/maintainers/portfile-functions.html -> /en/docs/maintainers/ports/vcpkg-gn/vcpkg_gn_install.html
Broken fragment link /en/docs/maintainers/pr-review-checklist.html -> /en/docs/maintainers/maintainer-guide.html#Avoid-deprecated-helper-functions
Broken fragment link /en/docs/maintainers/pr-review-checklist.html -> /en/docs/maintainers/maintainer-guide.html#Avoid-excessive-comments-in-portfiles
Broken internal link /en/docs/maintainers/vcpkg_configure_gn.html -> /en/docs/maintainers/ports/vcpkg-gn/vcpkg_gn_configure.html
Broken internal link /en/docs/maintainers/vcpkg_install_gn.html -> /en/docs/maintainers/ports/vcpkg-gn/vcpkg_gn_install.html

I intend to address these issues in the main vcpkg repository, then regenerate the website and add this as a mandatory validation step to prevent future regressions.