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ARROW-17887: [R][Doc] Improve readability of the Get Started and README pages (#14514)

This pull request proposes a number of changes to the pkgdown site for the R package:

- Reduces the content on the README page to the essential points
- Rewrites the "get started" page to focus on common tasks and novice users
- Moves discussion of the Arrow data object hierarchy to a new "data objects" vignette
- Moves discussion of Arrow data types and conversions to a new "data types" vignette
- Moves discussion of schemas and storage of R attributes to a new "metadata" vignette
- Moves discussion of package naming conventions to a new "package conventions" vignette
- Moves discussion of read/write capabilities to a new "reading and writing data" vignette
- Moves discussion of the dplyr back end to a new "data wrangling" vignette
- Edits the "multi-file data sets" vignette to improve readability and to minimize risk of novice users unintentionally downloading the 70GB NYC taxi data by copy/paste errors
- Minor edits to the "python" vignette to improve readability
- Minor edits to the "cloud storage" vignette to improve readability
- Minor edits to the "flight" vignette to improve readability
- Inserts a new "data object layout" vignette (in the developer vignettes) to bridge between the R documentation and the Arrow specification page

In addition there are some structural changes:

- The pkgdown template now uses bootstrap 5
- The articles menu organizes the vignettes into meaningful categories
- The former "project docs" menu has been replaced with a sidebar on the main page
- ~~Some vignette filenames have been edited and links updated~~ (this has been reverted)
- ~~The developer vignettes are now in the main vignettes folder~~ (this has been reverted)

Possible issues as yet unaddressed: 

- I have not yet checked whether the bootstrap 5 template breaks the script inserting the documentation versions switcher
- Changes to developer vignettes are extremely minimal in comparison to other vignettes. I'm uncertain whether to make further changes there or to defer that to a later PR
- The "articles" menu currently hides all the developer vignettes under the "more articles" link: they should be made more prominent and easy to find
- Some topics may not be described as well as we'd like? 
- The proposed edits to the vignettes include a lot more code that is executed at build time. This may not be desirable?


Authored-by: Danielle Navarro <djnavarro@protonmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>