Update documentation for next release of EOS Online Subsystem

• Document systems/presence.md "Presence concepts"
• Document systems/presence.md "Updating a local user's presence"
• Document systems/presence.md "Retrieving presence of another player"
• Document systems/presence.md "Get notified when another player's presence changes"
• Document systems/presence.md "Access a local user's friend code"
• Document systems/presence.md "Resolve a friend code into a user ID"
• Document systems/presence.md "Open a low-frequency reliable message channel to another player"
• Update systems/identity.md "Hooking into the identity system" to reflect the new identity hooks
• Document presence_updating.mdx "Setting up localisation", and add a link to it from systems/presence.md
• Update presence_updating.mdx blueprint example for localisable text keys
• Clarify where presence is seen in presence_updating.mdx