Add tip for step argument conversions to NullValueRetriever docs

[SeanKilleen]

Closes #2619.

Remaining Tasks

• Add tip
• Find right format to link to https://docs.specflow.org/projects/specflow/en/latest/Bindings/Step-Argument-Conversions.html
• Update doc with correct link
• Ensure the tip preview will display as expected to the best of my ability
• Add changelog entry. It does not appear that docs-only changes get an entry in the changelog, so I skipped this.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• Performance improvement
• Refactoring (so no functional change)
• Other (docs, build config, etc)

Checklist:

• I've added tests for my code. (most of the time mandatory) (docs, not code)
• I have added an entry to the changelog. (mandatory) (does not seem to apply here)
• My change requires a change to the documentation. (well, it is a change to the documentation.)
• I have updated the documentation accordingly.

[SeanKilleen]

@SabotageAndi I think this covers the doc changes that would help with #2619 in the future. Ready for your review when you're able.

The ReadTheDocs preview failed, but I can't see why. I took my formatting from this ReadTheDocs Sphinx page but perhaps I did something wrong.

[SabotageAndi]

@SeanKilleen looks like something changed on ReadTheDocs side. Suddenly some configuration is missing.
I will have a look at it.

[SeanKilleen]

Great! Looks like it's passed now, though I don't have access to view it.