UID2-6642 Remove Identity Map restrictions and add Rate Limiting section

[swibi-ttd]

Context
The /identity/map endpoints are undergoing the following changes:

• Participants are no longer required to send requests to these endpoints sequentially
• Rate Limiting will be added to these endpoints, meaning participants sending too many requests will receive 429s

Documentation changes

• Remove restriction for participants to only send one /identity/map request at a time
• Add Rate Limiting section to /identity/map endpoint pages
• Add 429 to Response Status Codes table

Notes:

• The actual rate limit will not be disclosed to participants

[genwhittTTD]

Couple small mods. One is really just a question but I suspect I'm wrong. Checking anyway.

[genwhittTTD]

LGTM thanks.