Check for missing dependencies before building the website

[gunnarku]

Summary:

• Add an executable permission to the file.
• Check for the dependencies before the website build starts.
• Output a list of missing dependencies.
• Note: command -v is POSIX compatible.

Sample output:

$ ./build_website.sh
Command 'pydoc-markdown' not found.
Command 'python' not found.

Some of the dependencies are missing.
Please install them to build the website.

Why are these changes needed?

TL;DR: make the life of anyone who wants to contribute to the documentation (and doesn't necessary run all the steps described in README.md ;-)) easier. My workflow consisted of 5-6 retries till I got all the dependencies right. This set of changes reduces the frustration and performs the dependency checks beforehand.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [ X] I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (71829f3) 37.80% compared to head (1306610) 37.80%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1678   +/-   ##
=======================================
  Coverage   37.80%   37.80%           
=======================================
  Files          50       50           
  Lines        5726     5726           
  Branches     1296     1296           
=======================================
  Hits         2165     2165           
  Misses       3390     3390           
  Partials      171      171           

Flag	Coverage Δ
unittests	37.80% <ø> (ø)

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[jackgerrits]

Looks good, thanks!