Prepare for version 0.3.0

[shinich1]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run tox)
• If applicable, add a line to the [unreleased] part of CHANGELOG.md, following keep-a-changelog.
• Format added code by ruff
  • See CONTRIBUTING.md for more details
• Make sure the checks (github actions) pass.
• Check that the docs compile without errors (run make html in ./docs/ - you may need to install dependency for sphinx docs, see docs/requirements.txt.)

Then, please fill in below:

Context (if applicable):
For v0.3, we would like to:

• Deprecate non-core features to improve the maintainability.
• update docs
• stop supporting python 3.8
• switch to fastflow for flow-finding algorithms

Please add comments below for what we need to do for more organized package. Please create PRs to be merged into prepare-v03 branch, to propose further changes.

Description of the change:

• more commits to follow until this is no longer a draft PR.

Related issue:

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 77.12%. Comparing base (7da2095) to head (3e2e76c).

Files with missing lines	Patch %	Lines
graphix/pattern.py	60.00%	4 Missing ⚠️
graphix/sim/statevec.py	33.33%	2 Missing ⚠️
graphix/sim/density_matrix.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
- Coverage   78.99%   77.12%   -1.88%     
==========================================
  Files          40       40              
  Lines        6113     5473     -640     
==========================================
- Hits         4829     4221     -608     
+ Misses       1284     1252      -32     

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