Spellcheck of enhancingperf.rst
Spellcheck of the docs, mostly `enhancingperf.rst`, and a sentence or two on `sparse.rst`.

* Added a short introductory paragraph on enhancing performance.
* Backticks ` `` ` around Series, DataFrame.
* Typeset variants of ` ``numba`` `, `(n|N)umba`, `(c|C)ython`, etc as **Numba** and **Cython**.
* Minor rephrasing of sentences, spelling, periods, colons etc.

Cheers. Comments very welcome :)

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19516?src=pr&el=h1) Report
> Merging [#19516](https://codecov.io/gh/pandas-dev/pandas/pull/19516?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/69cd5fbfd8b57033b0eb280ebc6dd502652f269c?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19516/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19516?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19516      +/-   ##
==========================================
- Coverage   91.69%   91.67%   -0.03%     
==========================================
  Files         148      148              
  Lines       48561    48561              
==========================================
- Hits        44530    44518      -12     
- Misses       4031     4043      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.03% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19516?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19516/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19516?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/19516?src=pr&el=footer). Last update [69cd5fb...5ed6c4a](https://codecov.io/gh/pandas-dev/pandas/pull/19516?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks again @tommyod 