Add newly documented crossposting related endpoints and models.

[FasterSpeeding]

Summary

Add newly documented crossposting related endpoints and models.

Checklist

• [x ] I have run nox and all the pipelines have passed.
• [ x] I have made unittests according to the code I have added/modified/deleted.

Related issues

[codecov]

Codecov Report

Merging #147 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   79.48%   79.53%   +0.04%     
==========================================
  Files          80       80              
  Lines       10096    10128      +32     
  Branches     1286     1288       +2     
==========================================
+ Hits         8025     8055      +30     
- Misses       2001     2003       +2     
  Partials       70       70              

Impacted Files	Coverage Δ
hikari/api/entity_factory.py	100.00% <ø> (ø)
hikari/api/rest.py	100.00% <ø> (ø)
hikari/channels.py	100.00% <100.00%> (ø)
hikari/guilds.py	95.12% <100.00%> (+0.01%)	⬆️
hikari/impl/entity_factory.py	100.00% <100.00%> (ø)
hikari/impl/rest.py	89.09% <100.00%> (+0.11%)	⬆️
hikari/utilities/routes.py	100.00% <100.00%> (ø)
hikari/impl/bot.py	32.73% <0.00%> (-0.24%)	⬇️
hikari/impl/shard.py	78.60% <0.00%> (ø)

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[davfsa]

LGTM 👍

[FasterSpeeding]

Gonna add some methods to the Follow object real quick before merging

[ghost]

Looks really good, but I have a few questions and suggestions at the cost of being nitpicky :)

[ghost]

ping me when updated and ill look again.