[Documentation] Fix Broken/Missing Links in Token Cache Documentation

[jmprieur]

In docs/token-cache/README.md, there are references and links to files such as in-memory.md and possibly others (e.g., serialization, distributed-cache) that are not present in the feature/doc-modernization branch. This creates confusion and dead-ends for readers seeking in-depth coverage as described.

Required Fixes:

• Create missing in-memory and (if warranted) distributed-cache, serialization subpages as stubs or fully documented pages
• Alternatively, consolidate all intended content in the README if dedicated subpages will not be shipped, and remove or update stale links
• Ensure all internal documentation links resolve correctly

PRs must target: feature/doc-modernization branch.

Affected file: docs/token-cache/README.md and possibly new subpages