NEWS.md: add release notes for v0.35.0

[grondo]

This is a rough draft of release notes for v0.35.0.

This assumes a couple pending PRs will go in. I added a short blurb about "better handling of situations where is broker is unresponsive but not disconnected", but I'm assuming @garlick will edit that for correctness and clarity.

Feel free to push any edits or additions directly.

[garlick]

Pushed a little update to the blurb at the top.

[garlick]

approving per offline discussion with @grondo

[codecov]

Codecov Report

Merging #4103 (5a6d721) into master (a85ab39) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4103      +/-   ##
==========================================
+ Coverage   83.83%   83.85%   +0.01%     
==========================================
  Files         374      374              
  Lines       54818    54818              
==========================================
+ Hits        45959    45965       +6     
+ Misses       8859     8853       -6     

Impacted Files	Coverage Δ
src/common/libpmi/simple_client.c	86.33% <0.00%> (-3.28%)	⬇️
src/common/libpmi/pmi.c	91.50% <0.00%> (-1.89%)	⬇️
src/broker/overlay.c	88.44% <0.00%> (-0.61%)	⬇️
src/cmd/flux-mini.py	91.86% <0.00%> (-0.15%)	⬇️
src/common/libflux/message.c	94.60% <0.00%> (-0.14%)	⬇️
src/broker/state_machine.c	82.14% <0.00%> (+0.51%)	⬆️
src/modules/job-archive/job-archive.c	60.08% <0.00%> (+0.80%)	⬆️
src/common/libsubprocess/server.c	77.01% <0.00%> (+4.88%)	⬆️