[ZEPPELIN-5549] Shiro authentication doc improvements
[ZEPPELIN-5549] Shiro authentication doc improvements

### What is this PR for?
- Adds note about Knox SSO using Hadoop Groups for group mappings (copied and modified from SPNEGO).
- Corrects markdown formatting issues in SPNEGO section.
- Modified the wording of some SPNEGO instructions for clarity.
- Fixes a few minor typos and grammar issues.

### What type of PR is it?
Documentation

### How should this be tested?
* Verify that markdown changes render correctly, especially those under the SPNEGO heading.

### Questions:
* If this PR looks good, could you kindly apply the 'hacktoberfest-accepted' label? 😅

Author: llamasoft <llamasoft@rm-rf.email>

Closes #4246 from llamasoft/ZEPPELIN-5549/shiro-jwt-docs and squashes the following commits:

a8377cd78 [llamasoft] Improve Hadoop group mapping wording
ff6db7ee1 [llamasoft] Fix typos in Shiro authentication docs
1b2a1ea2f [llamasoft] Fix SPNEGO markdown formatting
36741186f [llamasoft] Add Hadoop Groups note to Knox SSO docs

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