templates/login.html: fix 404s in offsite How to set a password docs, link to TLS/SSL if scheme!=https

[westurner]

Description

• The links to how to set a password are 404ing
  https://github.com/jupyter/nbclassic/blob/main/nbclassic/templates/login.html
• They could also link to how to TLS/SSL if scheme!=https
• These docs could be included in the app, so that they work without internet access

Reproduce

1. Cause a login failure e.g. by opening http://localhost:8888/?token=invalidtoken
2. 404: Click the " enable a password. " or " the documentation on how to enable a password " links

• https://jupyter-notebook.readthedocs.io/en/stable/public_server.html is gone?
• https://jupyter-notebook.readthedocs.io/en/v6.5.4/public_server.html remains
• DOC,SEC: also add a conditional link to #using-ssl-for-encrypted-communication
• DOC: include these docs in nbclassic/templates/login.html

Expected behavior

• the link to the offsite, online docs on how to set a password should not 404

Context

• Operating System and version:
• Browser and version:

• Jupyter NbClassic version:

Troubleshoot Output

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You may want to sanitize the paths in the output.

Command Line Output

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Browser Output

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[RRosio]

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