Add vignettes on algorithm arguments + options; update qgis_run_algorithm() docs

[florisvdh]

Two vignettes are added:

• an overview of R object classes that are accepted for each QGIS argument type, in tabular format. It is an attempt to summarize the outcomes of Complete input argument type parsing #13.
• an overview of available options & environment variables to configure package behaviour.

Whoever has some time, please review!

A convenient way to render them locally is by running:

pkgdown::init_site()
pkgdown::build_article("qgis_arguments")
pkgdown::preview_page("articles/qgis_arguments.html")
pkgdown::build_article("qgis_options")
pkgdown::preview_page("articles/qgis_options.html")

This might be included in a next package release before useR! 2024; some features were already added earlier this year.

[florisvdh]

• Still to be added (forgot to do so): qgis_output* classes (ed85db6).

[florisvdh]

• Also add a (shorter) respin of the paragraph 'For data objects in R that already exist as a stored file [...]' from the docs of qgis_run_algorithm().