DOC: create text.rst with string methods (GH8416)
closes #8416 

@jorisvandenbossche @hayd ?
 This looks like a great change - it's also reducing the rather unwieldy size of the "basics" section. So, +2.
 @davclark the question is what else, if anything should go in here? 
 To my knowledge, you've moved the _only_ narrative documentation about string types. So, there's nothing else that should be moved. A common point of confusion for beginners is the auto-regexp functionality in many of these methods. So a highly visible message about that point could be useful both in the "getting started" and at the top of this new section. But I think this should be a separate pull request if you want me to draft it, right?
 sure u can update this one
or push on top
 @jreback I sent you a pr to your str branch.
 @jorisvandenbossche ?
 Yep, also +1 on seperating this into a seperate file.

Added two minor comments. And I was also thinking, what I am missing at the moment is a subsection about the indexing capacity of `str`:

```
`In [1]: s = Series(['A', 'B', 'C', 'Aaba', 'Baca', np.nan, 'CABA', 'dog', 'cat']
)

In [2]: s
Out[2]:
0       A
1       B
2       C
3    Aaba
4    Baca
5     NaN
6    CABA
7     dog
8     cat
dtype: object

In [3]: s.str[0]
Out[3]:
0      A
1      B
2      C
3      A
4      B
5    NaN
6      C
7      d
8      c
dtype: object

In [4]: s.str[1]
Out[4]:
0    NaN
1    NaN
2    NaN
3      a
4      a
5    NaN
6      A
7      o
8      a
dtype: object
```
 @jorisvandenbossche ok, let me put this up and can add to it (I'll add something like your example as well).
