Rearrange README/vignette & check for duplicate file output

[RayStick]

Closes #195
References #184

In addition to closing #195, it also checks for the existence of the BAR_ plot and does not re-make if it finds it in the output_dir already (this is the same for every table run i.e. one BAR_ plot per dataset)

Checklist for the author of this PR:

• [if package files were edited] I have run these checks locally:
  • devtools::document() to generates the .Rd files from any updated roxygen comments.
  • lintr::lint_package(path = ".") to check for style compliance and syntax issues.
  • codemetar::write_codemeta() to ensures the metadata file is up to date.
  • devtools::check() for a comprehensive package check. I have resolved any warnings or errors, or written them here in the PR, for discussion.
• The code base and the documentation files match (they both reflect any recent changes).
• The title of this PR is clear and self-explantory.
• I have added any appropriate labels to this PR.
• This PR is now ready for review (and I have removed the draft PR status).

[codecov]

Codecov Report

Attention: Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
R/metadata_map.R	86.95%	3 Missing ⚠️

Files with missing lines	Coverage Δ
R/metadata_map.R	71.16% <86.95%> (-0.99%)	⬇️