Provide visibility to Graph Network indexes

[BigLep]

Done Criteria

There is visibility on docs.filecoin.io about ways a developer can get access to The Graph network indexes.

Why Important

Having access to these capabilities is critical for certain applications. Currently it isn't very visible to developers that Filecoin has these capabilities thanks to certain infra providers.

User/Customer

dapp developers who would benefit from The Graph network indexes

Notes

1. (I have no expertise here. I'm only documenting the need as I understand it. Please correct/fix what I am wrong about.)
2. Maybe expand https://docs.filecoin.io/networks/mainnet/rpcs with a separate section?
3. Even if all the current providers are private (I don't know if that is true or not), it would be good to surface that there is a way forward (and document what that would be). For example, maybe the steps are "fill in this form about your usecase and X will get back to you about your options" or "reach out to X on FIL Slack to learn more".

[BigLep]

Cc @eshon as I'm assuming you have feedback or ideas here.

[ArseniiPetrovich]

Made a #2336 PR