DOC: Use top-level pd.IntervalArray in doc examples
Follow-up to #23581

Minor changes to make sure `pd.IntervalArray` is used in the doc examples instead of `pd.core.arrays.IntervalArray`.  Pretty sure this is all the instances of them; I tried to be diligent in marking these with a TODO, and can't find any additional examples via a quick search.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24475?src=pr&el=h1) Report
> Merging [#24475](https://codecov.io/gh/pandas-dev/pandas/pull/24475?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ff280485a24acbb5f466f4e29d19ab7bc3d02de2?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24475/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24475?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24475   +/-   ##
=======================================
  Coverage   92.32%   92.32%           
=======================================
  Files         166      166           
  Lines       52298    52298           
=======================================
  Hits        48285    48285           
  Misses       4013     4013
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.74% <ø> (ø)` | :arrow_up: |
| #single | `43.06% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24475?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/24475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2ludGVydmFsLnB5) | `93.04% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/24475/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `95.26% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24475?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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