84 benchmarking page

[nabe98]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run tox)
• format added code by black -l 120 <filename>
• If applicable, add a line to the [unreleased] part of CHANGELOG.md, following keep-a-changelog.

Then, please fill in below:

Context (if applicable): In the current implementation, there is no benchmarking page although there were some benchmarking examples (e.g. ones on https://github.com/TeamGraphix/graphix/pull/74, https://github.com/TeamGraphix/graphix/pull/43, memory usage of TN simulation, https://github.com/TeamGraphix/graphix/pull/81, etc)

Description of the change:

• Add benchmarks directory to store sample codes for benchmarking
• Add benchmarking code (statevec.py) for remove_qubit
  • graphix vs paddle_quantum vs mentpy with reference circuit simulator on graphix

Related issue:

84

also see that checks (github actions) pass. If lint check keeps failing, try installing black==22.8.0 as behavior seems to vary across versions.

[shinich1]

@nabe98 I see the readthedocs build is now working, great! Do you want me to review this PR?