NHS barcode and action links on /features message channel pages

[emmagifford]

Description

As part of this PR, I have:

• added new content on NHS barcodes under a H3 header in the 'Envelopes' section at the bottom of /features/letters
• added new action links on the 4 message channel pages in /features to the respective Word message template pages in /using-nhs-notify - see 'What you need to know' sections

Context

1. Royal Mail are now offering NHS barcodes to help them identify NHS mail and prioritise it during busy periods. NHS Notify letters will automatically have this barcode on the envelope.

Sommer and Gaby asked us to add some generic content outlining this. We need to keep the guidance quite basic as Royal Mail may update their webpage over time, so linking out to this covers us.

2. The action link changes come as a result of Emily's suggestion in our Teams chat. Emily mentioned that she'd expect to be able to easily find the Word template pages if she was browsing the message channel pages in /features - this meets her mental model. It's a fair point and a good idea so I added the links in.

Sian and I felt that an action link (as opposed to inset text or a bulleted link) was the best component to use to make this stand out more.

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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