Revise admin area Pages settings UI

What does this MR do?

Revises the UI text of the admin area Pages preferences.

For the general OKR issue technical-writing#419 (closed)

Closes #330235 (closed)

Screenshots or Screencasts (strongly suggested)

I moved the two checkboxes that didn't have a heading to the top to make them more apparent, as previously they were buried between max size and Let's Encrypt settings.

before	after

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.