Pre-8.4.2 release changelog and doc fixes

[kdeldycke]

Inconsistencies I spotted after 8.4.1 release:

• Missing PR references in changelog entries:
  • Handle empty bytes in echo #3493
  • fix I/O operation on closed file error with CliRunner and echo_via_pager #3482
  • Fix get_parameter_source() during type conversion and eager callbacks #3484
  • shell autocomplete uses normalize_choice()  #3471
  • Fix 2994 launch locate files w spaces windows #3478
• Changelog style and formatting
• .. versionadded:: admonition styling
• rST to Markdown rewrite format

This is a WIP PR that I will use along the stable branch development cycle and is to be merged before its release.

Before removing the draft status, review all changes in the pending release diff: 8.4.1...stable

Follow up on #3495.

[kdeldycke]

@Rowlando13 is the empty header element introduced at https://github.com/pallets/click/pull/3447/changes#diff-b4d68dc855d0f9476d3f2ee343853bd21bf82ea9960d0cf06661baa244439dd6R1 intentional? In which case I backported the original rST markup in this PR with 86d6015

[Rowlando13]

Yes. The Sphinx role that was used before is either not supported or has a bug in myst. It is to not have a header on the landing page.

[Rowlando13]

But looks like you fixed it properly so all good.

[kdeldycke]

I rebased this PR, so I guess you can merge it if you think we are approaching the 8.4.2 release. The only left over are typing releases, so no changelog required for these.