update top-level docs

[aappling-usgs]

This PR updates several top-level docs, primarily for the sake of tidiness and modernity:

• Switch recommended PAT handling from Sys.setenv to gitcreds functions. I did some experimenting and suspect gitcreds isn't even necessary for all these public repos, but I'm keeping it in because they are necessary for devtools::build_readme() (as now noted in dev.R) and I'm not sure they're NOT necessary for some users; in particular, we still don't have a straightforward installation figured out for Greg, though he found a workaround.
• Add vignette links to ?loadflex and README.md
• Use "_PACKAGE" correctly in loadflex.R, which causes pkgdown to move that documentation from man/loadflex.Rd to the now more standard man/loadflex-package.Rd
• Move startupMessage contents into ?loadflex, because it feels less valuable and more obnoxious these days
• Use standard, pkgdown-expected header format in NEWS (e.g., # Version 1.9.3)

[github-actions]

📖 pkgdown preview for this PR: https://doi-usgs.github.io/loadflex/preview/pr263/

[gfkltn]

Admin review completed. No issues found.