Tidy up before releasing stable v1.0 version

[cjrace]

Creating this issue as a catch all for a variety of smaller tasks, mostly clean up, that I'd like to consider doing before properly calling it v1.0.0 on CRAN and committing to a stable base. Where breaking changes are brought in we'll need to keep backwards compatability of some kind and use proper lifecycle warnings to help users in the short term.

Can update the list below for now, and then break into separate issues to pick up when the time comes.

Current list (still very much open to suggestions or thoughts!):

• Swap insert_text to inset_text (seems like a typo and I want to make it match GOV.UK's component)
• Change the argument names in header to fit their current usage better, e.g. dashboard_title instead of secondary_text, which consistently confuses new users
• Review gov_layout and remove if no longer recommended (or change to something like gov_simple if we decide we want to keep a simple variant)
• ...