CCM-2973 new web UI features quick fixes

[emmagifford]

Description

As part of this PR, I've:

• moved the 3 'by use case' pages to /features under a new heading in the left-hand nav, 'Features by messaging needs'
• changed the character limit of NHS App messages from 1,000 to 5,000 in /features/nhs-app-messages
• clarified that links in letters do not need https:// at the start in /using-nhs-notify/links-and-urls
• updated all 5 of the blank letter templates - made the personalisation section more prominent, added personalisation fields that match the fields in the upcoming example personalisation csv to help users visualise them, removed https:// from the example links

Context

Moving the 'by use case' list of features to /features is a quick win as part of CCM-12973. We believe this will help users to find the content more easily and understand the product offering depending on their particular use case, at a glance. I updated the term 'by use case' to 'by messaging needs' as this is a better use of plain language.

The letter link updates were prompted by user queries, documented in CCM-12976.

The NHS App message character limit change is needed to fix the incorrect content - spotted by Emily!

Type of changes

• Refactoring (non-breaking change)
• 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

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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