sagemathgh-37878: Run sage doc server for jupyterlab 
    
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<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
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Reopen sagemath#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 sagemath#34794.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37878
Reported by: Kwankyu Lee
Reviewer(s): Kwankyu Lee, Marc Culler

sagemathgh-37878: Run sage doc server for jupyterlab 
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

Reopen sagemath#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 sagemath#34794.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37878
Reported by: Kwankyu Lee
Reviewer(s): Kwankyu Lee, Marc Culler