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[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 75.41%. Comparing base (90c9433) to head (7ecf33c). Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
graphix/pattern.py	62.50%	3 Missing :warning:
graphix/graphsim/rxgraphviews.py	66.66%	1 Missing :warning:
graphix/sim/density_matrix.py	50.00%	1 Missing :warning:
graphix/sim/statevec.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #218 +/- ## ========================================== - Coverage 77.41% 75.41% -2.00% ========================================== Files 38 38 Lines 6163 5520 -643 ========================================== - Hits 4771 4163 -608 + Misses 1392 1357 -35 ```

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