[R] Update install.Rmd vignette

[asfimport]

Proposed changes:

• Break up further to make more skimmable, by using more subheadings

Reporter: Nicola Crane / @thisisnic
Assignee: Nicola Crane / @thisisnic

Related issues:

• [R] Mention conda in the Linux installation guide (is duplicated by)

PRs and other links:

• GitHub Pull Request #11521

_{Note: This issue was originally created as ARROW-13398. Please see the migration documentation for further details.}

[asfimport]

Nicola Crane / @thisisnic:
Issue resolved by pull request 11521
#11521