Update contributing.md

[virginiascarlett]

We should eventually update contributing.md to reflect myst, not hugo. I just did this myself, so I'm jotting down what I did for reference, in case it's helpful.

1. Fork and clone the repo, set up remotes if needed. (Already done)
2. Create a virtual environment, i.e. a .venv folder
   --> I use uv for python package management, so I did uv venv. I think if you were using venv, you would do python -m venv .venv.
3. Install dependencies
   --> I already had node and npm globally installed on my mac with homebrew. I installed the stuff in requirements.txt via uv like so: uv pip install -r requirements.txt. Then I installed myst like so: uv pip install mystmd. (We should probably add node, npm, and myst to requirements.txt? I didn't want to mess with it because I didn't know if these can't or shouldn't be added for some reason.)
4. Restart your IDE and activate the environment
   --> VS Code offered to associate this env with this project, and I said yes, so now it automatically loads this env when I open this folder in VS Code. Nice. Otherwise, you'll have to type out source .venv/bin/activate (macOS/Linux) or .venv\Scripts\activate (Windows) at the start of each session.
5. Preview the site
   --> Serve locally with myst start. Don't do myst start from root! cd into jbook/ and THEN do myst start. _build/ files are in .gitignore, so doesn't matter whether you're on main or a feature branch--start serving locally from wherever.

[LauraLangdon]

This all came in with my PR to move from Jupyter Book to MyST a bit ago, I think: #228