Add upgrading to 4.x docs

[j1m-ryan]

Proposed changes

Adds upgrading to 4.x docs page
Fix some small aspects of the create license secret page (linking to nim, some header changes)

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[github-actions]

✅ Deploy Preview will be available once build job completes!

Name	Link
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.64%. Comparing base (df9b043) to head (e90efce).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6930      +/-   ##
==========================================
- Coverage   52.95%   52.64%   -0.31%     
==========================================
  Files          88       88              
  Lines       20623    20758     +135     
==========================================
+ Hits        10921    10929       +8     
- Misses       9255     9372     +117     
- Partials      447      457      +10     

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[ADubhlaoich]

LGTM generally.

I'd consider moving this document into the installation folder, and linking to it from the release page where relevant.

There's text in the structured logging section that's repeated that could be put outside of the tabs: right now that structure feels a bit weird because of how everything is laid out.

A numbered list suggests a sequence of steps: point 3 is a piece of information, not an instruction, and point 4 is an optional step depending on the environment. If it's not a critical step necessary for all users, we prefer to give optional steps their own section.

Although it might feel slightly excessive due to the length of the document, converting some of the steps into subsections would demarcate the steps easily and make formatting easier: there's a pattern in our documentation I dislike where everything is a list item.

List items are all indented and the gymnastics around making sure it formats and displays properly mean you're incentivized to write as little as possible.

[ADubhlaoich]

Blocking this temporarily: I'm going to push some changes directly to the branch, discussed with @j1m-ryan .

[ADubhlaoich]

Adding an approval following my direct edits.

The commit message provides the context of the changes:

This commit updates the information architecture and metadata
for documentation related to NGINX Ingress Controller 4.0.0.

It clarifies the relationship between certain documents, moves or hides
pages and delineates sections of the upgrade documentation while
making the text instruction more concise and precise.

There are some contextual changes that might warrant extra explanation: using discrete sections instead of itemizing the instructions in a single list improves the readability and sequence of steps involved.

The license secret documention was moved to its position to reflect that the instructions are contextless, and might be involved with a different use case/path than just upgrading or installing NGINX Ingress Controller (such as building it yourself).