Update pandas.read_gbq docs to point to pandas-gbq
The `pandas-gbq` package must be installed to use `pandas.read_gbq`.
Also, with soon-to-be-release version 0.3.0 of `pandas-gbq` the Google
Cloud client library is used instead of the Google API library.

Also, standard SQL is no longer beta. In fact it is highly recommended
over using legacy SQL.

- `N/A` - closes #xxxx
- `N/A` (docs change only) - tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- `N/A` (docs change only) - whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=h1) Report
> Merging [#18548](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2a0e54bc841f27164b116135ebda4b74bae2fc4a?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18548/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18548      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.02%     
==========================================
  Files         164      164              
  Lines       49802    49802              
==========================================
- Hits        45496    45487       -9     
- Misses       4306     4315       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.13% <ø> (ø)` | :arrow_up: |
| #single | `40.81% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <ø> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=footer). Last update [2a0e54b...a1ce420](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=h1) Report
> Merging [#18548](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7627ccaf9442f4101afda69b6077e7f035e23543?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18548/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18548      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.02%     
==========================================
  Files         164      164              
  Lines       49802    49802              
==========================================
- Hits        45496    45487       -9     
- Misses       4306     4315       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.13% <ø> (ø)` | :arrow_up: |
| #single | `40.81% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <ø> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=footer). Last update [7627cca...135d2f8](https://codecov.io/gh/pandas-dev/pandas/pull/18548?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Since this PR no longer talks about explicit dependencies that `pandas-gbq` has it is not dependent on https://github.com/pydata/pandas-gbq/pull/25. I believe it can be merged any time. thanks @tswast 