docs: add introduction to pandas users for adding columns with single text values (literal) (ibis-project#11010)

## Description of changes

ibis generally follows pandas syntax for mutating columns, with the
exception of adding string columns where `ibis.literal` is needed.
`ibis.literal` is not mentioned yet in the pandas' user introductions,
so it would be helpful to include this as it's not clear to the user why
adding a float column or summing two columns together uses the same
syntax as pandas, while a adding a single-valued string column does not.

This is the reason
ibis-project#11009 was opened (as a
q&a link to send to new users).

Including it in the pandas introduction docs would reduce friction for
switching to ibis for pandas users.

Author: ScottWilliamAnderson

docs/tutorials/coming-from/pandas.qmd

@@ -234,6 +234,12 @@ mutated = t.mutate(two=t.two * 2)
 mutated
 ```
 
+To add a column with a single text value on all rows, you can use `ibis.literal`.
+
+```{python}
+mutated = t.mutate(two_string=ibis.literal('two'))
+```
+
 ### Renaming columns
 
 In addition to replacing columns, you can rename them as well. This is done with