Restructure for web UI roll out

[emmagifford]

Description

This update incorporates the new IA for /using-nhs-notify.
As part of this update, I've:

• removed the 3 pages for the digital channels, which contained the old Microsoft Word docs
• grouped together guidance on formatting + links and URLs (rather than theming by message channel)
• reframed the letter templates guidance to be more about uploading a letter and updated the page title
• added a new page, Create and submit a letter - this introduces the concept of templates and acts as the new start page for web UI, replacing the old one - it has a sign in button that links to the sign in page
• changed 'routing plan' terminology to 'message plans' and changed one instance of 'fallback' to 'fall back' where it's used as a verb on the old /routing-plans page (now /message-plans)

Context

This update is required because the release of MVP web UI for all users means that the template process is changing.
GOV Notify structure their formatting guidance in this way, rather than by message channel, so we know it must be tested and it works.

For the formatting page, I tried to set out which features are available for each channel. After Alex's request, I also clarified where a feature isn't supported by a channel.

I added links to other relevant pages to help users navigate the content.

For letters, I decided to keep most of the formatting guidance on the /upload-a-letter page, because this uses Word styles rather than Markdown, so it made more sense to group it together on that page with the downloadable templates. However,

I added anchor links to these specific sections from the /formatting page.
CCM-10606 is the ticket for this.

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

---

Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.

[jakecosgrove]

Everything looks good here, well done on the whole raft of changes!