[DOC] Document the archival blockchain data backup sources

[frol]

Is your feature request related to a problem? Please describe.

There are backups provided by various providers and we need to make them discoverable:

• Allnodes: https://www.publicnode.com/snapshots#near
• Pagoda: https://near-nodes.io/intro/node-data-snapshots

Describe the solution you'd like

I think RPC and "Data on Blockchain" pages can be extended with this information