Improve formating, sectioning, etc in general edit of docs

[hturner]

In recent version we have built up the online docs: https://contributor.r-project.org/r-dev-env/.

The quality could be improved with a general edit. Some particular issues I have noticed:

1. Inconsistency in list/section style

   Some sections use enumerated lists, e.g. creating codespace, some use 1) style numbering which is not recognised as a markdown list, e.g. Running R, some use this style in subsection headers, e.g. updating source code. It would be good to have more consistency, using proper markdown enumerated lists or unnumbered sections where appropriate. Note that subsections get added to the navigation - not sure what the limit is, but even level 4 headers are added - and these allow you to use anchored links, which can be helpful, e.g. Close R terminal. It would also be good to add permalinks to the section headers, perhaps with a link symbol.

2. Issues in page/section headers

   Sometimes page headers are (effectively) duplicated, e.g. Start Workspace, other times they need separating out from list/sub-section headers, e.g.
   1. Example Contribution Workflow using the R Dev Container: in the R Contribution Workflow section is really a title for the whole page, not just item 1 of the list.

3. Incorrect indentation

   We mostly caught this in the initial PR review, but there are still some stray examples, e.g. the > Before edit paragraph in the Editing Source Code section needs to be indented so that it aligns under point 2 in the list.

Having some fresh eyes review and edit the documentation as a whole would be very helpful!

[hturner]

Taking advantage of further features of mkdocs could also come into this general edit, e.g. can we add search to the docs?