Adds for loop example to docs

[joaomcteixeira]

Every time I use pdb-tools I always feel it is an amazing tool. :heart_eyes: I believe extra examples can help the users understand the capabilities of the pdb-tools design.

This PR adds a little, yet powerful, example with bash for loop.

[codecov[bot]]

Codecov Report

Merging #95 (7a85271) into master (5c7039e) will not change coverage. The diff coverage is n/a.

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  Hits         3004     3004           
  Misses        469      469           
  Partials      190      190           

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[JoaoRodrigues]

Following this example, we could have a cookbook with "recipes" for combining the tools. That could help newcomers get started?

[joaomcteixeira]

I agree, @JoaoRodrigues Actually, the submitted example is a tentative to seed that. Shall all those examples go to the current section or should there be a specific section of combination examples? Wherever it should go, once we decide on that, we can simply continue PR examples as we use them in our daily work.

[JoaoRodrigues]

We can add a COOKBOOK.md or an EXAMPLES.md file that simply lists examples in list. We'd have to link it to the index.md file so that it shows up in the web page and include this one in a setup.cfg file so that it is shipped with the code.

[joaomcteixeira]

Cookbook added in #97