docs: improve scannability of chain data sources

[yash-blip-ys]

This PR improves the readability of the 'Modularity' section in the README. I have converted the chain data source list into a nested list to make it easier for new developers to see the supported backends at a glance hope i added little value to our world

[ldk-reviews-bot]

I've assigned @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[yash-blip-ys]

Hey team I noticed that the VSS Integration Tests failed, while 10 other checks—including Documentation, Python Tests, and multiple Rust builds—have passed successfully.
I investigated the VSS failure logs and it appears to be an environment-related issue:
PostgreSQL 'root' role mismatch.
A Tokio runtime panic in vss_read_write_remove_list_persist_in_runtime_context.
Since my changes are strictly formatting improvements in the README.md, these failures appear unrelated to this PR. I’ll continue to monitor the remaining pending checks

[tnull]

We don't accept trivial PRs like this that don't add any value and can be considered noise (plus the change itself is a net negative, IMO). Please refrain from opening them.

[yash-blip-ys]

sorry sir understood my fault still on path of learning will try to learn and add value next time