Improve UX for warnings arounds doc config and generation

[starbelly]

This PR closes #263

With a doc directory that contains an index.html file but no doc provider configuration :

Publishing truecoat 0.4.56 to hexpm
  Description: It gets installed at the factory
  Dependencies:

  Included files:
    LICENSE
    README.md
    rebar.config
    rebar.lock
    something_else/foo
    src/truecoat.app.src
    src/truecoat.erl
  Licenses: Apache 2.0
  Links:

  Build tools: rebar3
Be aware, you are publishing to the public Hexpm repository.
Before publishing, please read Hex CoC: https://hex.pm/policies/codeofconduct
Proceed? ("Y")>
Local Password:
===> Published truecoat 0.4.56
===> Published docs for truecoat 0.4.56

Without doc provider configuration and no doc dir with an index.html file found :

Publishing truecoat 0.4.59 to hexpm
  Description: It gets installed at the factory
  Dependencies:

  Included files:
    LICENSE
    README.md
    rebar.config
    rebar.lock
    something_else/foo
    src/truecoat.app.src
    src/truecoat.erl
  Licenses: Apache 2.0
  Links:

  Build tools: rebar3
Be aware, you are publishing to the public Hexpm repository.
Before publishing, please read Hex CoC: https://hex.pm/policies/codeofconduct
Proceed? ("Y")>
Local Password:
===> Published truecoat 0.4.59
===> No valid hex docs configuration found. Docs will will not be generated.
===> Docs were not published since they couldn't be found in '/Users/starbelly/devel/erlang/truecoat/doc'. Please build the docs and then run `rebar3 hex docs` to publish them.