docs(readme): move the reference material into a published docs site #175

Problem Statement

The README is the only documentation. It shows one request, and says nothing about the accepted input, the error responses, which providers answer, how the cache matches locations, the command-line flags or the metrics. Some of what it does say does not match the code:

CONTRIBUTING.md points contributors at the README for interface changes, and names a tagging.yml workflow that does not exist.

Proposed Solution

Publish a docs site with MkDocs on Read the Docs, and cut the README to a description, a quick start and a table of pages.

nav:
  - Overview: README.md
  - Installation: installation.md
  - Configuration: configuration.md
  - API: api.md
  - Architecture: architecture.md
  - Metrics: metrics.md

Affected Resources

• README.md - cut down, with absolute links, because the chart release packages it
• docs/ - new pages
• mkdocs.yml and .readthedocs.yaml - new site and build configuration
• CONTRIBUTING.md - the interface step and the versioning step
• .gitignore - the MkDocs build output

Resource Links

• https://docs.readthedocs.com/platform/stable/config-file/v2.html

Acceptance Criteria

• Read the Docs builds the site from main.
• Each page states only behaviour the code shows.
• The README links work on GitHub and on Artifact Hub.
• CONTRIBUTING.md names only files and workflows that exist.