Add template for Design Principles vignette

[joshwlambert]

This PR adds a vignette to the {packagetemplate} which will ensure that each new package generated with this template contains the a design principles vignette. As outlined in the vignette template, the sections are optional and the vignette template is a high-level guide to Epiverse-TRACE package developers.

These changes add package vignette infrastructure to the {packagetemplate} including, additions to the .gitignore, vignettes/.gitignore and DESCRIPTION.

These changes are a result of development to try and get Epiverse-TRACE packages communicating more developer focused decisions, an overview can be found in the Epiverse-TRACE discussions.

[joshwlambert]

One thing that came up in discussion around the design vignette was whether suggested dependencies should be included. In the {simulist} vignette they are all listed, however, we agreed it would be best if only uncommon suggested dependencies were mentioned. Some common suggested dependencies are: {knitr}, {rmarkdown}, {ggplot2}, {testthat}. This leads to the question of where to document that we have this list of default package infrastructure suggested dependencies. Would it be useful to add a paragraph to blueprints?

[pratikunterwegs]

Thanks for opening this PR @joshwlambert, looks good to me. We should mention this in today's developer meeting as well.

[Bisaloo]

Thanks! I like it! It's comprehensive but flexible and not overwhelming. Good work!

[joshwlambert]

One thing I didn't change in this PR which I have done in packages with a design vignette is added a specific header to the vignette drop-down menu on the pkgdown site (by editing _pkgdown.yml). I will not do this in this PR as I think it overcomplicates it for developers that use the template. If someone thinks it is required I can add it in a separate PR.