Lift keyword-containing text from Starlark docs to `ytt` docs to increase hit rate / decrease time to answer

**Describe the problem/challenge you have**
Documentation about core Starlark language functionality is pointed too (and not copied) and doesn't show up in search results.

Exhibit A:
> Sorry for the repeat offenses, team. I'm having some trouble getting string concatenation working and a search on the docs yields nothing.


**Describe the solution you'd like**
Select portions of Starlark docs that include likely keywords and copy into `ytt` docs (retaining link to the source).

**Anything else you would like to add:**
The example given is _just_ an example. Consider:
- what other actions one would take on Starlark-native values
- what types does that action apply.

To extend _this_ example: not only can strings be concatenated, but so can lists, tuples, and dictionaries.

Thanks goes to @drawsmcgraw for noting this deficiency. 🙏🏻

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