DOC: add read_gbq as top-level in api.rst
Noticed that the link to the top-level one did not work in the whatsnew file
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=h1) Report
> Merging [#16256](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a31c96d34d00dc757908b564dc93991e867d83e2?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16256/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16256      +/-   ##
==========================================
- Coverage   90.33%   90.33%   -0.01%     
==========================================
  Files         164      164              
  Lines       50890    50890              
==========================================
- Hits        45972    45969       -3     
- Misses       4918     4921       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.11% <ø> (+0.01%)` | :arrow_up: |
| #single | `40.3% <ø> (-0.1%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.58% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |
| [pandas/\_version.py](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=tree#diff-cGFuZGFzL192ZXJzaW9uLnB5) | `44.65% <0%> (+1.9%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16256?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/16256?src=pr&el=footer). Last update [a31c96d...74785b5](https://codecov.io/gh/pandas-dev/pandas/pull/16256?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
