Remove Pin.depth and add meta examples

[lidel]

This PR

• moves depth from being a mandatory field to optional meta attributes. (as per feedback in https://github.com/ipfs/pinning-services-api-spec/pull/16) Removed as per https://github.com/ipfs/pinning-services-api-spec/pull/19#discussion_r453685376
• documents various uses of meta fields, including peering hints to speed up transfer of data that is not already on the pinning service (closes #18, as per feedback in https://github.com/ipfs/pinning-services-api-spec/issues/18#issuecomment-656778692)

cc @obo20 @achingbrain

[lidel]

Thank you for feedback!

• I applied clarifications/suggestions
• Opened https://github.com/ipfs/pinning-services-api-spec/issues/22 to discuss promotion of peering hints to mandatory fields
• Merging this to remove depth from shared spec – feel free to follow-up in new issues / PRs