DEPR: Deprecate cdate_range and merge into bdate_range
- [X] closes #17596
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

A bit going on here, so see summary below.

Deprecated `cdate_range`:
- Removed from api (was added to api after 0.20.3 release)
- Added a `FutureWarning` to `cdate_range` indicating deprecation
    - Added a test for this too
- Converted other tests using `cdate_range` to use `bdate_range` instead
    - Cleaned up the entire test script to make it more consistent with others (i.e. `with` context)
- Removed a reference to `cdate_range` from the current whatsnew (previous commit I wrote)

Expanded functionality of `bdate_range`:
- Added the additional `cdate_range` params (`weekmask`, `holidays`) for custom frequency ranges
- Expanded that functionality beyond what was previously present in `cdate_range`
    - `cdate_range` only supported cday with the `weekmask` and `holidays` params, now all custom freqs are supported.
    - Did this by checking if the passed freq starts with 'C'.
    - Added a test that should catch if a new non-custom freq is added that starts with 'C'.
    - Didn't add this to `cdate_range`; left that as-is.
- Added a `UserWarning` if non-default `weekmask` or `holidays` are passed without a custom freq.
    - Added a test for this too.

Edits to `timeseries.rst`:
- Added a custom freq `bdate_range` example to the "Generating Ranges of Timestamps" section
- Reordered some of the examples in the "Generating Ranges of Timestamps" section
    - Tried to order examples by simplicity
- Otherwise, mostly small rewording changes and other such small fixes (ticks, capitalization, etc.).

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=h1) Report
> Merging [#17691](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/408ecd21759979f94952a48a0e4a2b2a608ee84d?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17691/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17691      +/-   ##
==========================================
- Coverage   91.23%   91.21%   -0.02%     
==========================================
  Files         163      163              
  Lines       49796    49806      +10     
==========================================
+ Hits        45430    45431       +1     
- Misses       4366     4375       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <100%> (ø)` | :arrow_up: |
| #single | `40.33% <30.76%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.14% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.58% <100%> (+0.04%)` | :arrow_up: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17691?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/17691?src=pr&el=footer). Last update [408ecd2...4cbf70d](https://codecov.io/gh/pandas-dev/pandas/pull/17691?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Made the review changes and fixed some unrelated typos I noticed in whatsnew (missing : and `). Made the additional review changes. lgtm. @jorisvandenbossche @TomAugspurger if you have anything further. thanks @jschendel 