DOC: Clean up of API doc sections
Follow up of the splitting of `api.rst` in #24462.

- Making better use of the titles and subtitles (in date offsets, the class name, and the properties and methods sections where at the same level for example).
- We were inconsistently using `Attributes` and `Properties`, left just properties, as it was the most common one.
- Renamed `indices.rst` to `indexing.rst`, I think it makes more sense (`index.rst` is kind of reserved to the home)

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24508?src=pr&el=h1) Report
> Merging [#24508](https://codecov.io/gh/pandas-dev/pandas/pull/24508?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/43c4dcd45a50de04658dc78c7f2e5c62eaf6b5d2?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/24508/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24508?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24508      +/-   ##
==========================================
- Coverage   31.91%    31.9%   -0.02%     
==========================================
  Files         166      166              
  Lines       52400    52428      +28     
==========================================
+ Hits        16722    16725       +3     
- Misses      35678    35703      +25
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.29% <ø> (-0.02%)` | :arrow_down: |
| #single | `31.9% <ø> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24508?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/24508/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `27.95% <0%> (-0.42%)` | :arrow_down: |
| [pandas/core/dtypes/inference.py](https://codecov.io/gh/pandas-dev/pandas/pull/24508/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2luZmVyZW5jZS5weQ==) | `80% <0%> (+0.96%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24508?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/24508?src=pr&el=footer). Last update [43c4dcd...96d9671](https://codecov.io/gh/pandas-dev/pandas/pull/24508?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/24508?src=pr&el=h1) Report
> Merging [#24508](https://codecov.io/gh/pandas-dev/pandas/pull/24508?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/43c4dcd45a50de04658dc78c7f2e5c62eaf6b5d2?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/24508/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24508?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24508      +/-   ##
==========================================
- Coverage   31.91%    31.9%   -0.02%     
==========================================
  Files         166      166              
  Lines       52400    52428      +28     
==========================================
+ Hits        16722    16725       +3     
- Misses      35678    35703      +25
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.29% <ø> (-0.02%)` | :arrow_down: |
| #single | `31.9% <ø> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24508?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/24508/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `27.95% <0%> (-0.42%)` | :arrow_down: |
| [pandas/core/dtypes/inference.py](https://codecov.io/gh/pandas-dev/pandas/pull/24508/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2luZmVyZW5jZS5weQ==) | `80% <0%> (+0.96%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24508?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/24508?src=pr&el=footer). Last update [43c4dcd...96d9671](https://codecov.io/gh/pandas-dev/pandas/pull/24508?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. Thanks @datapythonista !