[AIRFLOW-3656] Show doc link for the current installed version

[kaxil]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-3656

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Jira explains it well: https://issues.apache.org/jira/browse/AIRFLOW-3656

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  added

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6690 into master will decrease coverage by 0.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #6690      +/-   ##
==========================================
- Coverage   83.83%   83.82%   -0.02%     
==========================================
  Files         668      668              
  Lines       37543    37546       +3     
==========================================
- Hits        31476    31474       -2     
- Misses       6067     6072       +5

Impacted Files	Coverage Δ
airflow/www/app.py	95.76% <75%> (-0.76%)	⬇️
airflow/jobs/local_task_job.py	86.25% <0%> (-3.75%)	⬇️
airflow/utils/dag_processing.py	57.99% <0%> (-0.17%)	⬇️

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[feluelle]

LOL we didn't had that? I haven't noticed that the whole time - probably because I am always using the latest version :D

QoL change :)