Rollup merge of #84451 - torhovland:flex, r=jsha
Rollup merge of #84451 - torhovland:flex, r=jsha

Use flex more consistently

Builds on #84376, related to #84354.

- Fully replaces `float: right` with `flex` on `.content .out-of-band`.
- Uses `flex` more consistently with existing usage (on `h3`, `h4`, etc.).

Tested on various widths to make sure the pages behave as before.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust