R Client HTML Docs

[alexpeters1208]

Remaining work for the R Client lies in getting the source documentation into an HTML website, discoverable from deephaven.io. This will be broken into the following tasks:

1. Get HTML doc source reviewed and merged (DONE in 4782)
2. Get gradle to generate the non-check-ables from the HTML doc source, to generate the .html files (DONE in 4782)
3. Get the .html files that are produced hooked into deephaven.io like the other client docs (4851)

The first item falls on me (and Chip for review), the second may be a me-Cristian-Colin problem, and the third sounds like a me-Don-Devin problem.

Additionally, the new docsite is made much more navigable when the docs previously called rdeephaven.Rd, AggBy.Rd, and UpdateBy.Rd are moved to vignettes. These are longer-form documentation and belong in a separate tab on the website. Making this change introduces the problem that install.packages(...build_vignettes = TRUE) does not build and install the vignettes when the package is built from source. R CMD build and R CMD INSTALL do take care of the vignette build and install, so we need to move the gradle builds and README to use these commands. This will happen in two steps.

1. Verify that these commands can be made to return a non-zero exit status, so that gradle picks up on failure. (DONE in 4782)
2. Change the build scripts and README to make this the "proper" method of installation. (DONE in 4782)

I will link PRs here as they are opened.