Replace 104 leftover ReadMe.io doc: scheme links across six branches

[allister-beamable]

104 links across six branches still use ReadMe.io's internal doc: URL scheme, which MkDocs does not resolve. They render as dead links.

Example, unity/v5.1 docs/unity/user-reference/beamable-services/identity/identity.md:

see the documentation on the [Player-Centric API](doc:beamable-player-centric-api)

Counts by branch:

Branch	Count
unity/v5.1	43
unity/v6.0	43
unreal/v2.2	6
unreal/v2.3	6
core/v7.1	6
core/v7.2	6

api/v1.0, websdk/v1.0, toolkit/v0.4, and internal are clean.

Find them with:

grep -rn '](doc:' docs --include='*.md'

Notes for whoever takes this:

• The unity/v5.1 and v6.0 sets are the same 43 links, since v6.0 was branched from v5.1 and is byte-identical apart from the version table — fix once, apply to both
• Same for the unreal pair
• The core/* six must be fixed from a core worktree. Check whether they sit under docs/cli/guides/ or docs/includes/; if so they are core-owned paths and a downstream edit would be silently discarded by auto-sync-core
• Each doc: slug maps to a legacy ReadMe page, so resolving them needs the same slug→new-path mapping the ReadMe sunset work needs. Worth doing after that mapping exists rather than guessing 104 targets independently

Not bite-size at 43 links per Unity branch, though the unreal and core sets might each be one session.