docs: add a simple example for using a single package in a devshell

[thenbe]

I was trying to add a single package from nur to a devshell, but it wasn't immediately clear to me how to do so. This PR demostrates adds a simple example for that use case.

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The following points apply when adding a new repository to repos.json

• I ran ./bin/nur format-manifest after updating repos.json (We will use the same script in github actions to make sure we keep the format consistent)
• By including this repository in NUR I give permission to license the
  content under the MIT license.

Clarification where license should apply:
The license above does not apply to the packages built by the Nix Packages
collection, merely to the package descriptions (i.e., Nix expressions, build
scripts, etc.). It also might not apply to patches included in Nixpkgs, which
may be derivative works of the packages to which they apply. The aforementioned
artifacts are all covered by the licenses of the respective packages.

[Pandapip1]

LGTM

[Pandapip1]

@mergify queue

[mergify]

queue

❌ Pull request must be rebased manually

The pull request can't be rebased without conflict and must be rebased manually

[Pandapip1]

@mergify queue

[mergify]

queue

🛑 The pull request has been removed from the queue default

The pull request #732 has been manually updated.

You can take a look at Queue: Embarked in merge queue check runs for more details.

In case of a failure due to a flaky test, you should first retrigger the CI.
Then, re-embark the pull request into the merge queue by posting the comment
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[Pandapip1]

@mergify refresh

[mergify]

refresh

✅ Pull request refreshed

[Pandapip1]

Not sure why mergify isn't liking this PR.

@mergify queue