Feature/bcss 20436 calendar picker util markdown doc and docstrings (#48)

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## Description

<!-- Describe your changes in detail. -->
Adding a markdown documents detailing how to use the calendar picker
util.
Added doc-strings to all of the methods in the calendar picker util.
Also includes changes the the calendar picker util that were created in
[PR 46](#46)

## Context

<!-- Why is this change required? What problem does it solve? -->
Allows for the team to understand how to use the util and each method by
either reading the markdown document or the doc strings under each
method.

## Type of changes

<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply. -->

- [x] Refactoring (non-breaking change)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would change existing
functionality)
- [ ] Bug fix (non-breaking change which fixes an issue)

## Checklist

<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->

- [x] I am familiar with the [contributing
guidelines](https://github.com/nhs-england-tools/playwright-python-blueprint/blob/main/CONTRIBUTING.md)
- [x] I have followed the code style of the project
- [ ] I have added tests to cover my changes (where appropriate)
- [x] I have updated the documentation accordingly
- [ ] This PR is a result of pair or mob programming

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Author: adrianoaru-nhs