Add conda recipe and update installation instructions

[hdrake]

This pull request introduces several improvements to the packaging and installation process for the sectionate project. The most significant changes are the addition of a conda recipe for distribution, updates to installation instructions, and enhancements to CI and documentation environments.

Packaging and distribution:

• Added a new conda recipe in conda/meta.yaml for the sectionate package, specifying dependencies, build instructions, and metadata to enable conda-forge distribution.

Installation instructions and documentation:

• Updated README.md to replace the Binder badge with PyPI, Conda, Docs, and License badges, and changed the recommended installation command from pip install sectionate to conda install sectionate.

Continuous integration environment:

• Modified ci/environment.yml to add the nodefaults channel, ensuring only conda-forge packages are used for testing.

Documentation environment:

• Updated docs/environment.yml to include documentation-related dependencies: sphinx, nbsphinx, ipykernel, and pip, improving support for building docs and running notebooks.