Issue #535: Create pkgdown docs for stable and dev versions

[sbfnk]

Description

This PR closes #535.

It does so by setting the development version (four components) as done in the usethis package.

It also adds a note for the user to clarify which version of the documentation they're looking at. This is mostly to help people arriving from a search engine.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title as follows: issue-number: PR title
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• I have built the package locally and run rebuilt docs using roxygen2.
• My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (52e2cb8) 84.55% compared to head (55e178b) 84.55%.

❗ Current head 55e178b differs from pull request most recent head 0214000. Consider uploading reports for the commit 0214000 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #550   +/-   ##
========================================
  Coverage    84.55%   84.55%           
========================================
  Files           21       21           
  Lines         1722     1722           
========================================
  Hits          1456     1456           
  Misses         266      266           

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[seabbs]

LGTM. Actions user your alive!