Update actions and pkgdown site

[dieghernan]

Related with #8

This PR adds actions for testing on the following platforms;

        config:
          - {os: windows-latest, r: 'devel'}
          - {os: windows-latest, r: 'release'}
          - {os: windows-latest, r: 'oldrel'}
          # - {os: macOS-latest, r: 'devel'}
          - {os: macOS-latest, r: 'release'}
          - {os: macOS-latest, r: 'oldrel'}
          - {os: ubuntu-latest, r: 'release'}
          - {os: ubuntu-latest, r: 'devel'}
          - {os: ubuntu-latest, r: '4.0'}
          - {os: ubuntu-latest, r: '3.6'}

So far eveything is working (back to R 3.6) as there is no really a test suite.

Additionally I added an action for building a pkgdown site with rogtemplate, see https://ropengov.github.io/rogtemplate/#option-a-deploy-using-github-actions

[dieghernan]

The sample pkgdown site: https://dieghernan.github.io/federalregister/