doc: fix a table in release notes
link and excerpt what announcements I can find

Release announcements sometimes include a brief summary, which is also
useful here.  Some of these announcement links could be improved by
linking them in the context of the hledger rather than ledger
list. There may be more more that gmane search didn't find.
doc: release notes announcements styling
doc: relnotes: make minor release headings smaller
doc: relnotes: unbold intra-release headings

to help the release headings stand out