Install guide: add signposting about chosen operation system and breadcrumb to guide start

[vickytnz]

Added:

• breadcrumb at top of pages of operating-model specific steps to link back to start of install guide https://prototype-kit.service-manual.nhs.uk/install/simple . Not added to system choice as this is a switcher page and has a link at the bottom to get back to the install home
• variable on steps to show operation system (windows or mac) set on the child pages with a nunjucks variable (this is used rather than the data from the radio buttons in case someone comes to the pages directly as choosing your operating system is only for triaging to the right pages). System added after steps to be reassurance but less repetitive than being at the beginning (Mac: Step 1 of 8)

The advanced guide is not changed as it is one level down from the get started guide and a single page so does not need extra breadcrumbs.

Description

Mac guide

Before	After

WIndows guide

Before	After

Checklist

• [ ] Tested against our testing policy (Resolution, Browser & Accessibility)
• [ ] CHANGELOG entry

[vickytnz]

Changes made to:

• make the variable operatingSystem for legibility
• remove the default text in case it's left out by accident
• changed the code to also work better if empty with the terse but effective if expression {{[":", operatingSystem ] | join(" ") if operatingSystem }}

Ideally i'd like to refactor these templates at some point to make some things more baked into the templates but not a priority.