arrow

GH-40591: [R] Add extra CSS for navbar on pkgdown website (#40610)



### Rationale for this change

This PR fixes https://github.com/apache/arrow/issues/40591. It adds some CSS rules for better readibility of text in the navbar on the `pkgdown` website.

### What changes are included in this PR?

Three new CSS rules, one for each change in the navbar:
* put the section text (e.g "Get started") in light gray. I chose light gray and not white because white is used when we hover the text;
* put the version number in light gray
* put the "Search for" placeholder in light gray

### Are these changes tested?

Not applicable

### Are there any user-facing changes?

Only visual changes on the website:

* before:

![image](https://github.com/apache/arrow/assets/52219252/54b7a47f-b997-464c-9787-12df3e3da541)

* after:

![image](https://github.com/apache/arrow/assets/52219252/0ccf878e-1607-4574-9b6c-f51804bdbe5b)

* GitHub Issue: #40591

Authored-by: Etienne Bacher <52219252+etiennebacher@users.noreply.github.com>
Signed-off-by: Bryce Mecum <petridish@gmail.com>