ENH: Added ability to redirect to external pages

[AakashGfude]

Added a config variable to specify redirects.

An Example:-

If in lecture-python, _config.yml file

redirects: 
        heavy_tails: 
          "url" : "file:///Users/u6533564/Documents/Office/Quantecon/lecture-python-intro/lectures/_build/html/heavy_tails.html"
          "redirect_text": "This page has been redirected from intro/heavy_tails.html"

Then, the page at heavy_tails in lecture-python will be redirected to the target URL, with information about where it is redirected from and the content in the URL. Like:-
?redirected_from=/Users/u6533564/Documents/Office/Quantecon/lecture-python.myst/lectures/_build/html/heavy_tails.html&content=This page has been redirected from intro/heavy_tails.html

The target page, on detecting the variables in the URL, displays the following admonition:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.07%. Comparing base (e65778b) to head (56fc4d2).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
- Coverage   73.18%   73.07%   -0.11%     
==========================================
  Files           2        2              
  Lines         261      260       -1     
==========================================
- Hits          191      190       -1     
  Misses         70       70              

Flag	Coverage Δ
pytests	73.07% <ø> (-0.11%)	⬇️

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[mmcky]

thanks @AakashGfude I will take a closer look at this soon.

So this detects incoming redirect and would display the admonition for any traffic coming from the old location?

[AakashGfude]

So this detects incoming redirect and would display the admonition for any traffic coming from the old location?

Yes, that's right @mmcky.