docs: Add documentation for the 'Create or edit Task' modal

[claremacrae]

Description

• Add new documentation page docs/getting-started/create-or-edit-task.md
• Update resources/screenshots/modal.png so that it shows the new tool tips for abbreviations of dates
• Updated Tasks plugin in sample vault to pre-1.8.0 release, to be able to update the screenshots of the Create/edit modal

Motivation and Context

• Fixes Add documentation for the 'Create or edit Task' modal #754
• Adds documentation for the abbreviation features added in feat: Add date abbreviations to 'Create or edit Task' modal #579

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which only improves the design or structure of existing code, and making no changes to its external behaviour)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-only change

Checklist:

• My code follows the code style of this project and passes yarn run lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change has adequate Unit Test coverage.

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