doc: add examples for implementing ESM

[rosaxxny]

Fixes: #28060

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

[addaleax]

@nodejs/modules-active-members

[GeoffreyBooth]

This PR seems to be adding an example explaining how import and export work in JavaScript in general, not particular to Node. I thought our general rule was that the Node docs only describe things specific to Node; they're not a general JavaScript tutorial like MDN. See #31479.

It's fine to add links to MDN, but we shouldn't be duplicating that resource.

[bmeck]

I think having a small example is fine, but if we could link to MDN like @GeoffreyBooth states as well it would give people a place to get a more in depth understanding.

[rosaxxny]

@GeoffreyBooth @bmeck There are links to MDN (for import and export) in the preceding paragraph.

@dnalborczyk @aduh95 Fixed.

Feedback is appreciated. Thanks!

[addaleax]

Landed in 4780493