Documentation Migration and Valkey Rebranding

[jeremyprime]

Summary

This PR migrates the existing Spring Data Redis docs from Antora (AsciiDoc) to Astro Starlight (Markdown), rebrands as Spring Data Valkey, and ensures Valkey GLIDE is the primary driver in the docs.

The styling from the Valkey GLIDE docs is applied to be consistent with other valkey.io docs.

All doc links in READMEs, schemas, etc. have also been updated.

Closes #14.

Review

See the rebranded docs for Spring Data Valkey here:
https://spring.valkey.io/

And compare the original Spring Data Redis docs:
https://docs.spring.io/spring-data/redis/reference/index.html

The overall structure was kept, but the pages were updated to fit the valkey.io styling and to make GLIDE the central driver in the docs.

Next Steps

Add a link under Java clients to point to the Spring Data Valkey repo and docs.