Gptsum

:construction_worker: Continuous Integration

* ci: add more flake8 linters (14) NicolasT

:books: Documentation

* docs: add quickstart to `README.rst` (15) NicolasT

:lipstick: Style

* ci: add more flake8 linters (14) NicolasT

:books: Documentation

* chore: add links to documentation (12) NicolasT

:construction_worker: Continuous Integration

* ci: build docs in `release` Action (10) NicolasT

* docs: expand `README.rst` (7) NicolasT
* gptsum: implement all CLI subcommands (6) NicolasT
* gptsum: implement `GPTImage` (5) NicolasT
* gptsum: add initial GPT header handling code (4) NicolasT
* Unify version handling and add CLI boilerplate (3) NicolasT
* chore: initialize project (1) NicolasT

:package: Dependencies

* chore(deps): update actions/checkout requirement to v2.3.4 (2) dependabot