add pkgdown website

[JosiahParry]

The pull requests creates a pkgdown site from the {oysteR} package. Note that I omitted functions audit_deps() and get_vulnerabilities() from the reference page as they are deprecated. There are no changes to the functionality of the package.

This pull request makes the following changes:

• Creates a pkgdown site for oysteR

Files & directories added:

• _pkgdown.yml: Metadata used to structure pkgdown site
• pkgdown/extra.css: Contains CSS used to style pkgdown site
• pkgdown/: Contains favicon's created by pkgdown .
• docs/: Contains html for pkgdown site

It relates to the following issue #s:

• Fixes https://github.com/sonatype-nexus-community/oysteR/issues/40

cc @bhamail / @DarthHater / @brittanybelle / @adrianpowell / @csgillespie

[csgillespie]

Added via #42 .

As we were using gh-pages, easier to extract sections for a new PR.

Thanks!