docs(date-range-picker): Include docs for DateRangepicker custom format

[AhsanAyaz]

Added the demo of custom format for date range picker
I've tried to find this many times, and probably others do to :)

PR Checklist

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• read and followed the CONTRIBUTING.md guide.
• [] built and tested the changes locally.
• [] added/updated tests.
• [] added/updated API documentation.
• [] added/updated demos.

[codecov]

Codecov Report

Merging #5199 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #5199   +/-   ##
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  Coverage        51.51%   51.51%           
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  Files                3        3           
  Lines               99       99           
  Branches            17       17           
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  Hits                51       51           
  Misses              38       38           
  Partials            10       10

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[codecov]

Codecov Report

Merging #5199 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #5199   +/-   ##
============================================
  Coverage        51.04%   51.04%           
============================================
  Files                3        3           
  Lines               96       96           
  Branches            17       17           
============================================
  Hits                49       49           
  Misses              37       37           
  Partials            10       10

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 42cc778...56a9ada. Read the comment docs.

[ludmilanesvitiy]

Need to update custom_date_format_spec.ts with

datepicker.isInputHaveAttrs(customFormat, [{ attr: 'formcontrolname', value: 'dateYMD' }], 0);
    datepicker.isInputHaveAttrs(customFormat, [{ attr: 'formcontrolname', value: 'dateMDY' }], 1);
    datepicker.isInputHaveAttrs(customFormat, [{ attr: 'formcontrolname', value: 'dateFull' }], 2);

[Domainv]

fixed

[ludmilanesvitiy]

Reviewed, looks good.