👷🏻‍♂️Automate the build and local preview of the website

[ebouchut]

This PR helps automate building and local previewing of the website.

ℹ️ ✅ Launch the below command and keep it running in a dedicated window/tab:

# cd trio-docs

sphinx-autobuild docs/EN docs/_build/html

It builds the website and runs a local Web server to preview the site live.
Each time you change a file, it rebuilds the website and hot-reloads the pages in the Web browser so that you can see the changes.
You will no longer need to reload the page you modified locally, sphinx-autobuild does it for you automatically.

See: https://pypi.org/project/sphinx-autobuild/

❌ You no longer need to rebuild the site each time you update a page:

# cd trio-docs

sphinx-build -M html docs/EN/ docs/_build/

• ➕ Add the sphinx-autobuild dependency.
• 📝 Add documentation on how to install trio-docs and the local preview of trio-docs