arrow

ARROW-14338: [Docs] Add version dropdown to the pkgdown (R) docs

**Implemented**:
- Version switcher replaces package version on pkgdown
- Package versions and URLs stored in `arrow/r/pkgdown/assets/versions.json`
- Tested with a web browser and JS console
- functionality to check URL can be retrieved and redirect to root dir if not (thanks @jorisvandenbossche for your previous work on this which constitutes 90% of this!)

![](https://user-images.githubusercontent.com/13715823/150108943-d58a0ca0-4840-4995-903b-3e9d4950705e.png)

**Suggestion for testing without putting together a complex Docker script:**
- copy script from `extra.js`
- as the `versions.json` file is not deployed yet, replace `./versions.json` in line `$.getJSON("./versions.json", function( data ) {` with `https://raw.githubusercontent.com/thisisnic/arrow/ARROW-14338_pkgdown_versioning/r/pkgdown/assets/versions.json`
- open R docs page and paste into JS console

Closes #12172 from thisisnic/ARROW-14338_pkgdown_versioning

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>