Issue 250: Ground work to shift development to main from develop

[seabbs]

Description

This PR closes #250.

As discussed in #250 there are a range of benefits to shifting development to main with the key benefits of using develop having been eroded by enabling release-only building on R-universe. This PR updates the README and contributing guide to drop mentions of develop and make distinctions clear between the various available package versions.

It also:

• Adds an action to automatically render the README.Rmd when it is touched.
• Fleshes out some of the install sections
• Adds a link out to our Docker container as an alternative.

This PR does not remove the develop triggers in our CI as this would break the current development workflow and does not remove the develop branch as we are still using it for this minor release.

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 in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have updated the package development version by one increment in both NEWS.md and the DESCRIPTION.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov]

Codecov Report

Merging #256 (08f1606) into develop (485c041) will not change coverage.
The diff coverage is n/a.

❗ Current head 08f1606 differs from pull request most recent head 1414b0f. Consider uploading reports for the commit 1414b0f to get more accurate results

@@           Coverage Diff            @@
##           develop     #256   +/-   ##
========================================
  Coverage    96.82%   96.82%           
========================================
  Files           15       15           
  Lines         1574     1574           
========================================
  Hits          1524     1524           
  Misses          50       50           

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[pearsonca]

Seems fine except: something to fix with the renderer?

A few other asides in comments; take 'em or leave 'em.