Add "doc proxy" that styles and alters broker-generated docs page

[jameshochadel]

Changes proposed in this pull request:

• docproxy is a small Go app that will be exposed as a public route like https://services.fr.cloud.gov. https://cloud.gov will link to it and it will gradually replace the parts of the services documentation that are references for the service plans.
• Docproxy makes the following changes to the docs page and could do more in the future:
  • Adds a favicon
  • Adds styles based on the USWDS
  • Removes leading whitespace from service titles
  • Sets a page title consistent with pages titles from https://cloud.gov
• This PR does not include a deployment method for the app yet. That will likely go in deploy-cf.

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

The new application will improve security because we will not need to expose the entire CSB API surface in order to make the documentation page public. It does not accept any user input.

[JasonTheMain]

Looks good, excited to see what it will looks like