Add Akavache Docs section

[ChrisPulman]

What are the main goals of this change?

• Better readability (style, grammar, spelling...)
• Content correction (accuracy, wrong information...)
• New content

This pull request adds comprehensive Akavache v11 documentation to the ReactiveUI handbook and updates related navigation and API docs to improve discoverability and usability. It introduces new guides covering installation, usage, serialization, settings, images, migration, performance, and troubleshooting for Akavache, and integrates these into the handbook’s structure and table of contents. The API documentation index is also restructured for clarity.

Akavache Documentation Additions:

• Added new handbook pages for Akavache v11, including guides for installation, usage, caches, serialization, settings, images, migration from v10, performance, and troubleshooting.

• Added a dedicated Akavache table of contents (akavache/toc.yml) for easy navigation of the new documentation.

Handbook and Navigation Updates:

• Updated the main handbook index to reference Akavache and reorganized platform and core topics for improved clarity.

• Integrated Akavache documentation into the handbook table of contents for seamless access.

API Documentation Improvements:

• Rewrote the API documentation index to clearly list and link all major ReactiveUI-related projects, including Akavache, and highlight .NET 8 compatibility.

[github-actions]

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