Improve documentation for `board_connect()`

[juliasilge]

Let's improve the docs for board_connect(), especially around how to use environment variables:

pins-python/pins/constructors.py

Lines 434 to 452 in ca9dde2

	Examples
	--------
	Use a server URL or set the `CONNECT_SERVER` environt variable to connect:
	```python
	server_url = "https://connect.rstudioservices.com"
	board = board_connect(server_url)
	```
	In order to read a public pin, use `board_url()` with the public pin URL:
	```python
	# for a pin at https://connect.rstudioservices.com/content/3004/
	board = board_url(
	"https://connect.rstudioservices.com/content",
	{"my_df": "3004/"}
	)
	board.pin_read("my_df")
	```

Internal users report they were not clear on how to actually set up their environment variables. Let's update this to be more like this vetiver demo where we show folks the code they really need to run.

Also note the typo to be fixed.

[juliasilge]

For R, we have the authentication details on the function reference page for board_connect() so I probably lean toward having it here, rather than as a section in a more general vignette.