Add a small link to RPC ecosystem providers

[janewang]

What problem does your feature solve?

I'd like a small link to the docs page to grab the mainnet RPC URLs: https://developers.stellar.org/docs/data/rpc/rpc-providers
Same for Horizon URLs: https://developers.stellar.org/docs/data/horizon/horizon-providers

The RPC is not pre-populated so it's really nice to have a link to that page. I know exactly where that page is so I navigate it to grab a RPC URL but I can see this is hard for someone who is less familiar with our dev docs structure, or someone new to our ecosystem.

Discussed: use an info icon