Fix sphinx warnings

[Robert-Steiner]

Issue

When building the documentation locally, some warnings appear. This pr fixes two of the 4 warnings.

Description

Warning 1:

checking consistency... /project/flower_github/doc/source/contributor-how-create-new-messages.rst: WARNING: document isn't included in any toctree

Fixed by renaming the filename.

Warning 2

/project/flower_github/src/py/flwr/simulation/app.py:docstring of flwr.simulation.app.start_simulation:24: WARNING: Inline interpreted text or phrase reference start-string without end-string.

Fixed by having no the line break within the backticks.

Related issues/PRs

Proposal

Explanation

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Update changelog
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?