Standardize header syntax within About section

[tetrapod00]

This is a proof of concept, before making changes to the whole online documentation.

This standardizes our RST header syntax as follows:

Page title
==========
Renders as h1. Every page has this.

Header
------
Renders as h2. Usually appears in sidebar. Many pages only need one level of nested headers.

Sub-header
~~~~~~~~~~
Renders as h3. Appears in sidebar in some pages, depending on how deeply nested the *page* is.

Sub-sub-header
^^^^^^^^^^^^^^
Renders as h4. Usually won't appear in the sidebar.

Sub-sub-sub-header
""""""""""""""""""
Renders as h5. If you got this far something went wrong. I'm not actually sure if we have any 5-layer header nesting in the docs.

Feel free to bikeshed the syntax order here, if you care. =, -, ~ is fairly common, but so is =, -, ^. """ is rarely used, but it does follow ^^^ when it is used. +++ is occasionally used.

Edit: after going through a lot more pages, =, -, ^ is common on pages with only 3 heading levels, but on pages with four heading levels, =, -, ~, ^ is standard, and =, -, ^, ~ or any variant is almost never used. So I think my original guideline is still correct.

Note that RST assigns headers based on the order they appear, within a particular page. So this PR will not change the header level of any existing pages. It only standardizes our syntax. This also means that this PR is not a one-to-one replacement of particular characters with other particular characters. It depends on the existing page!

Approval of this PR indicates the header syntax that I will standardize the rest of the docs with, in #10370.