Incorporate "Writing Custom Table Providers in Apache DataFusion" into the main datafusion docs #21304

Is your feature request related to a problem or challenge?

We have a very cool blog about table providers here: https://datafusion.apache.org/blog/2026/03/31/writing-table-providers/

Now that it is published, it would be great to incorporate a bunch of its content into the https://datafusion.apache.org/library-user-guide/custom-table-providers.html section of the doc

Originally posted by @alamb in apache/datafusion-site#161 (review)

Describe the solution you'd like

Specifically,

1. update https://datafusion.apache.org/library-user-guide/custom-table-providers.html with the content from the blog
2. Make examples compile (test with cargo test -p datafusion --doc)
3. Leave a link to the blog

Describe alternatives you've considered

No response

Additional context

No response