Document the HTTP action provider

[kurtmckee]

This branch adds documentation for the upcoming HTTP action provider, including:

• The URL and scope
• Complete documentation of all input document keys and values
• Examples of usage
• Documentation of the errors that may be returned by the HTTP action provider, including suggestions for how to resolve the errors.

[sc-11704]

[urimandujano]

These docs read really well and set the goal for where we want all our AP documentation to get to. I'd personally use this as a template to fill in the details on the other APs. My comments are opinions on reducing duplicated information but if you feel strongly about keeping as is I'm ok with that.

[kurtmckee]

Note that the changes in b6defdd depend on https://github.com/globusonline/globus-automate/pull/218.

[urimandujano]

Thanks Kurt! Thanks for mentioning that this relies on #218 -- let's hold off on merging this to master until we cut that PR in a release, otherwise we'll be out of sync with what is actually deployed.

[kurtmckee]

Updated to remove references to the user-configurable timeout key (which is no longer supported) and to document the new HTTP response size and total timeout errors.

[kurtmckee]

The HTTP action provider is currently for internal use. Once this restriction is lifted this can be reviewed and merged.