content: increase doc & click-target font-size; limit width of module tab
content: increase doc & click-target font-size; limit width of module tab

This CL modifies stylesheet.css to:

* Increase font-size for the search click targets (from 14px to 18px).
* Increase font-size for documentation.
* Set max-width for the module tab to 620px.

Fixes b/131862035
Fixes b/132199225
Fixes b/134426802

Change-Id: I01b0438b8e279e03ed8bcf49f8ef553659d769df
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/488895
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite