[Solonnikov]

Adds Octopin (https://octopin.app) to the two pinning service provider lists:

• docs/concepts/persistence.md
• docs/how-to/work-with-pinning-services.md

Octopin supports the IPFS Pinning Service API at https://api.octopin.app/psa, so it works with ipfs pin remote and the IPFS Desktop / Web UI pinning settings described on the how-to page. Docs: https://octopin.app/docs/pinning-api

It has a free tier (1 GB, no card required), so readers can try it the same way as the other listed providers.

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