docs: Add warning for unsupported time units in pandas pre v2

[benrutter]

What type of PR is this? (check all applicable)

• 💾 Refactor
• [ish] ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Related issue warn or raise on explicit incompatible TimeUnit #2964
• Closes warn or raise on explicit incompatible TimeUnit #2964

Checklist

• Code follows style guide (ruff)
• [-] Tests added (this happens within a pragma no cover block - I'm happy to take off the no-cover and add a test, but I think it potentially could require a pre-v2 pandas tests, or some clever patching?)
• Documented the changes

If you have comments or can explain your changes, please do so below

In order to tell between a default "us" and a specified "us", I've added in a _defaulted object variable to DateTime. This feels a little crufty, but I'm not sure I can see a better option?

[benrutter]

⚠️ Note that might be important in a subtle way that I haven't realised ⚠️

In order to make this consistent type wise, I've actually altered the narwhals/v1 type signature for dtypes. This feels like it could be a red-flag depending how type-strict folks our? Essentially, a None value can now (and is by default) be passed to time_unit, which under the hood switched to us but indicates that the time_unit has been set by default rather than explicitly.

I'm pretty sure this behaviour doesn't actually change anything for anyone's code, and it definitely doesn't invalidate any existing code, but editing stable/v1 makes me feel nervous 😅