GEN-108: Add a simple example for new error-stack users starting new project

[dpc]

Fix #3209

🌟 What is the purpose of this PR?

As a ever-re-returning user that wants to start a new project
using error-stack, I open http://docs.rs/error-stack and get
lots of words explaining some details, and not a single basic
example that would get me going.

This PR adds the simple example code that I'm looking always
looking for.

🛡 What tests cover this?

cargo test --doc :)

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #3228 (5b07a07) into main (069f9c9) will increase coverage by 1.06%.
Report is 6 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff            @@
##            main   #3228      +/-   ##
========================================
+ Coverage   1.23%   2.30%   +1.06%     
========================================
  Files        112     120       +8     
  Lines       3792    4038     +246     
  Branches     456     495      +39     
========================================
+ Hits          47      93      +46     
- Misses      3743    3939     +196     
- Partials       2       6       +4     

Flag	Coverage Δ
unit-tests	1.34% <ø> (+1.34%)	⬆️

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see 8 files with indirect coverage changes

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[TimDiekmann]

Thank you very much for your contribution. I think I'm sold on using thiserror in this context, otherwise, 4 additional impl blocks would have been required.
Happy to approve as soon as linting is passing (rustfmt is currently failing).

[dpc]

Thank you. Fixed (passes locally).

[TimDiekmann]

Amazing, thank you!

We can merge the PR as soon as the CLA is signed.

Btw, there is no need to force-push to rewrite the history of a PR, we squash merge anyway to keep a clean history in main.

[dpc]

We can merge the PR as soon as the CLA is signed.

Oh. I missed that. Done.