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 Replace Pin.depth with Pin.mode string enum #16) Removed as per Remove Pin.depth and add meta examples #19 (comment)
• documents various uses of meta fields, including peering hints to speed up transfer of data that is not already on the pinning service (closes Networking difficulties while pinning data #18, as per feedback in Networking difficulties while pinning data #18 (comment))

cc @obo20 @achingbrain

[lidel]

Thank you for feedback!

• I applied clarifications/suggestions
• Opened Mandatory provider hints #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