Add vignettes & update README

[jamesmbaazam]

In this PR,

• We move Introduction.Rmd vignette to the Quickstart section of README
• We add a detailed use-case vignette to resolve Add use case vignettes #12.
• We remove contexts from the test suite to fix Remove context() from tests #34.
• We add a website to fix Create pkgdown site for bpmodels #21.

[pratikunterwegs]

Thanks for tagging me @jamesmbaazam, leaving some general comments here.

Overall looks okay - the vignettes build and the website renders for me - but the main step is to bring this branch up to date with main - this will need to be done with a rebase as for previous branches. Once that is done this branch will integrate CI checks and should give a better picture what more is needed. Adding a render-readme workflow might be useful. Locally, R CMD check fails for me with a couple of errors and notes. I've left some comments on the files; happy to take another look or help with the rebase.

1. The 'introduction.Rmd' vignette seems to be a repeat of the Readme. Could some more information be added that would make this more useful/comprehensive/not the same?
2. Check the website version of the documentation using pkgdown::build_site() to check that the articles render in the intended order. If you want to separate the 'bp_literature' article into a separate section that needs to be done in a pkgdown.yml file.
3. Suggest a {styler} run (including on the vignettes, use style_dir()) and then a lint check as well. Also suggest using {goodpractice}.

[jamesmbaazam]

1. The 'introduction.Rmd' vignette seems to be a repeat of the Readme.

It's the same because it was moved to the README and is now deleted.

[jamesmbaazam]

Thank you for your comments and suggestions. I have now implemented all the changes.