docs: update docstrings and readme pystac-client

[stellamzr]

What I'm changing

• replace request with httpx information in docstring
• update docstrings
• add readme with examples

resolves #63

Checklist

• Tests pass: uv run pytest
• Checks pass: uv run pre-commit --all-files

[gadomski]

A couple of quick changes, and then a higher-level comment ... I've pushed a commit to this branch to add it to the mkdocs configuration ... this can be used to tell us if there's more things to fix up in the docstrings.

To run mkdocs:

uv run mkdocs serve

[gadomski]

I'm a little wary about including this table, as we'll have to keep it continually updated. I think I'd prefer to just point to the API docs, which will be generated from the code?

[gadomski]

Don't need this, since in the examples we're hitting a dummy endpoint.

Suggested change

Pre-request: The app should be accessible at `http://localhost:8000`.

[gadomski]

Nit: let's make the call simpler, to show you don't have to pass by keyword:

Suggested change

	product = client.get_product(product_id="test-spotlight")
	# List all Opportunities for a Product
	opportunities = client.get_product_opportunities(product_id="test-spotlight")
	# List orders
	orders = client.get_orders()
	# Get specific order
	order = client.get_order(order_id="test-order")
	product = client.get_product("test-spotlight")
	# List all Opportunities for a Product
	opportunities = client.get_product_opportunities("test-spotlight")
	# List orders
	orders = client.get_orders()
	# Get specific order
	order = client.get_order("test-order")