Add release notes for version 3.0

[oyvindeide]

Issue Resolves #my_issue

Approach Short description of the approach

Pre review checklist

• [ ] Added appropriate release note label
• [ ] PR title captures the intent of the changes, and is fitting for release notes.
• [ ] Commit history is consistent and clean, in line with the contribution guidelines.

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[codecov-commenter]

Codecov Report

Merging #3838 (898359b) into main (d20ddc3) will increase coverage by 0.41%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3838      +/-   ##
==========================================
+ Coverage   63.43%   63.85%   +0.41%     
==========================================
  Files         600      595       -5     
  Lines       45359    44580     -779     
  Branches     4091     4012      -79     
==========================================
- Hits        28774    28467     -307     
+ Misses      15328    14873     -455     
+ Partials     1257     1240      -17     

Impacted Files	Coverage Δ
..._c_wrappers/enkf/observations/block_data_config.py	57.14% <0.00%> (-21.43%)	:arrow_down:
src/clib/lib/enkf/field_config.cpp	1.35% <0.00%> (-13.70%)	:arrow_down:
src/ert/_c_wrappers/analysis/analysis_module.py	78.57% <0.00%> (-6.96%)	:arrow_down:
src/ert/ert3/engine/_sensitivity.py	88.63% <0.00%> (-6.37%)	:arrow_down:
src/clib/lib/enkf/enkf_obs.cpp	59.43% <0.00%> (-4.80%)	:arrow_down:
src/ert/ert3/engine/_export.py	89.55% <0.00%> (-3.90%)	:arrow_down:
src/ert/_c_wrappers/job_queue/workflow_job.py	82.55% <0.00%> (-3.27%)	:arrow_down:
src/ert/gui/ertwidgets/models/ertmodel.py	50.00% <0.00%> (-1.73%)	:arrow_down:
.../ertwidgets/models/analysismodulevariablesmodel.py	76.47% <0.00%> (-1.66%)	:arrow_down:
src/ert/ert3/engine/_run.py	95.23% <0.00%> (-1.51%)	:arrow_down:
... and 190 more

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