Add public declarations

[mcabbott]

First commit adds only those in api.md, but perhaps we should add more... like Dual?

help?> ForwardDiff.Dual
  │ Warning
  │
  │  The following bindings may be internal; they may change or be
  │  removed in future versions:
  │
  │    •  ForwardDiff.Dual

(eval is following this https://github.com/JuliaLang/julia/pull/55097/files )

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.96%. Comparing base (c310fb5) to head (e978ce2).
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #744      +/-   ##
==========================================
- Coverage   89.57%   86.96%   -2.62%     
==========================================
  Files          11       10       -1     
  Lines         969      959      -10     
==========================================
- Hits          868      834      -34     
- Misses        101      125      +24     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gdalle]

This is a very welcome initiative to guarantee API stability! So many people have been using ForwardDiff internals that they're not really internals anymore at this point.
For instance, here is the list of names that DI uses:

https://github.com/JuliaDiff/DifferentiationInterface.jl/blob/ba8a2fa9e7558e65655aeedbc36158e41ea5e324/DifferentiationInterface/ext/DifferentiationInterfaceForwardDiffExt/DifferentiationInterfaceForwardDiffExt.jl#L7-L28