Chore: Remove poetry from docs and package build

[iLLiCiTiT]

Changelog Description

Poetry is no longer used. Documentation is built with plain pip and docs/requirements.txt, and the package is built with setuptools using python -m build.

Additional info

Documentation

• Added docs/requirements.txt with documentation dependencies (sphinx, revitron theme, pydocstyle and ayon_api runtime dependencies).
• Sphinx is pinned to >=5.3.0,<7 because revitron_sphinx_theme fails on Sphinx 7+ (it uses the removed style template field). poetry.lock previously pinned Sphinx 5.3.0.
• The revitron theme is pinned to the same commit that poetry.lock resolved.
• docs-publish.yml workflow no longer uses abatilo/actions-poetry.
• docs/*.bat scripts call the tools directly, so they use the currently active environment.

Package build

• Build backend in pyproject.toml changed from poetry-core to setuptools; all [tool.poetry] sections removed.
• Removed setup.py. Its metadata was already overridden by [project] in pyproject.toml during the build (setuptools warned install_requires overwritten), and it contained stale dependencies (appdirs).
• Removed poetry.lock.
• Dev dependencies (mock, pytest, pydocstyle) moved to a PEP 735 [dependency-groups] table (pip install --group dev, uv sync).
• python-publish.yml builds with python -m build instead of python setup.py sdist bdist_wheel; README updated accordingly.

I compared a wheel built the old way (setup.py) with the new one: same 38 files and same Requires-Dist. Only the Home-page and Author metadata fields are gone (they came only from setup.py); Author-email and Project-URL: Repository stay.

Testing notes:

1. Create a clean venv and run pip install -r docs/requirements.txt.
2. Run sphinx-apidoc -f -e -M -o ./docs/source/ ./ayon_api/ and sphinx-build -M html ./docs/source ./docs/build; the build should succeed.
3. Run pip install build and python -m build; wheel and sdist should be created in ./dist.
4. Install the wheel to a clean venv and import ayon_api.
5. python manage_version.py update --version 1.2.25 should still update pyproject.toml and ayon_api/version.py.

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