docs: add database backend and runtime feature flags to crate docs

[mansiverma897993]

Documentation Improvement

Added a dedicated section in crate-level documentation listing:

• Supported database backends and their feature flags
• Supported async runtime feature flags

This improves discoverability of required feature flags
for users enabling specific database connectors.

No runtime behavior changed.
Documentation only.

[Huliiiiii]

You need to run cargo readme to update README.md.

[mansiverma897993]

Okk let me do it

[Huliiiiii]

I just realized we already have a detailed doc here: https://www.sea-ql.org/SeaORM/docs/install-and-config/database-and-async-runtime/.

Not sure if we still need to add this to docs.rs. What do you think, @tyt2y3?

[mansiverma897993]

Thank you for pointing that out.
Since the documentation already exists on the website, I agree it may not be necessary to duplicate it in docs.rs.

I’m happy to adjust the PR based on your suggestion. @Huliiiiii @tyt2y3

[mansiverma897993]

@Huliiiiii Any update? How you want to adjust this PR.

[Huliiiiii]

I'd like to wait for @tyt2y3 to give their feedback before making further changes to this PR.