Improve registries documentation

[vicroms]

This PR contains major changes to the registry's documentation:

• Adds new sections to the registries concept article (preview)
  • Registry directory structure
  • Versions database
  • Built-in registry
  • Obtain git-tree SHA
  • Best practices for registries
• Changes the registries reference article (preview)
  • Remove conceptual sections
  • Add JSON schema documentation for baseline.json and version files
• Fix broken links generated by these changes

[BillyONeal]

The only thing this is 'request changes' over are the file format examples that are wrong, everything else is take it or leave it nitpicks.

[BillyONeal]

I think we should add something like:

The version / verison-semver / version-date / version-string and port-version match the values in the port retrieved by the git-tree. For example:

PS D:\vcpkg> Get-Content D:\vcpkg\versions\c-\curl.json -TotalCount 7
{
  "versions": [
    {
      "git-tree": "6ef1763f3cbe570d6378632c9b5793479c37fb07",
      "version": "8.11.0",
      "port-version": 1
    },
PS D:\vcpkg> git show 6ef1763f3cbe570d6378632c9b5793479c37fb07:vcpkg.json | Select-Object -First 5
{
  "name": "curl",
  "version": "8.11.0",
  "port-version": 1,
  "description": "A library for transferring data with URLs",

I'm torn on whether this is a 'must' situation. It's true that if a user is manually doing this they must ensure that they do this, but it's also true that we provide tooling that makes this an implementation detail most of the time.

[vicroms]

Maybe we can add this to the "Recommended practices" section in the conceptual article.

[learn-build-service-prod]

Learn Build status updates of commit e95db83:

✅ Validation status: passed

File	Status	Preview URL	Details
.markdownlint.jsonc	✅Succeeded
vcpkg/about/faq.md	✅Succeeded	View
vcpkg/commands/add-version.md	✅Succeeded	View
vcpkg/concepts/manifest-mode.md	✅Succeeded	View
vcpkg/concepts/overlay-ports.md	✅Succeeded	View
vcpkg/concepts/registries.md	✅Succeeded	View
vcpkg/consume/boost-versions.md	✅Succeeded	View
vcpkg/consume/classic-mode.md	✅Succeeded	View
vcpkg/consume/git-registries.md	✅Succeeded	View
vcpkg/consume/lock-package-versions.md	✅Succeeded	View
vcpkg/consume/manifest-mode.md	✅Succeeded	View
vcpkg/contributing/maintainer-guide.md	✅Succeeded	View
vcpkg/get_started/overview.md	✅Succeeded	View
vcpkg/maintainers/registries.md	✅Succeeded	View
vcpkg/reference/vcpkg-configuration-json.md	✅Succeeded	View
vcpkg/users/authentication.md	✅Succeeded	View
vcpkg/users/config-environment.md	✅Succeeded	View
vcpkg/users/examples/versioning.getting-started.md	✅Succeeded	View
vcpkg/users/versioning-troubleshooting.md	✅Succeeded	View

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