gh-37878: Run sage doc server for jupyterlab
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Reopen #35139 because of the renewed interest:
https://groups.google.com/g/sage-devel/c/kzSWB8ps7VA
New environment variables added to `sage.env`:
```
SAGE_DOC_SERVER_URL = var("SAGE_DOC_SERVER_URL")
SAGE_DOC_LOCAL_PORT = var("SAGE_DOC_LOCAL_PORT", "0")
```
When Jupyter notebook launches,
- If `SAGE_DOC_SERVER_URL` is set, the url is used;
- else if local sage documentation is installed, a doc http server with
port `SAGE_DOC_LOCAL_PORT` starts. If the port is 0 (the default), then
a random port is selected by the system and stored to the environment
variable;
- else the online official documentation website
https://doc.sagemath.org is used
for the help menu of the Jupyter notebook.
Fixes #34794.
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/37878
Reported by: Kwankyu Lee
Reviewer(s): Kwankyu Lee, Marc Culler
