Add changes to v6 page

Co-authored-by: Calvin Lau <calvin.lau@digital.cabinet-office.gov.uk>

Author: domoscargin

source/changes-to-govuk-frontend-v6/index.html.md.erb

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+---
+title: Changes to GOV.UK Frontend v6.0.0
+weight: 95
+---
+
+# Changes to GOV.UK Frontend v6.0.0
+
+The main purpose of v6 include changes to:
+
+- improve our Sass architecture
+- use an updated type scale to improve accessibility for small screens
+- update colours to use the web palette in the GOV.UK brand guidelines
+- improve the page template layout
+
+We’ve also removed a number of deprecated APIs, Sass variables and component options.
+
+## Migrating from v5 to v6
+
+Follow the guidance about [staying up to date with GOV.UK Frontend](https://frontend.design-system.service.gov.uk/staying-up-to-date/).
+
+When making the decision to migrate, consider your service's requirements and your ability to update Sass tooling and templates. In particular, you must update to [Dart Sass v1.79.0 or newer](https://sass-lang.com/dart-sass/) before upgrading to v6.
+
+## Benefits of updating to v6
+
+By updating to v6 you'll:
+
+- stay aligned with the GOV.UK look and feel: we've updated GOV.UK Frontend to use the web colour palette from the GOV.UK Brand Guidelines
+- benefit from a more consistent colour system that's used in certain components
+- get the latest accessibility improvements, based on ongoing testing and feedback from services across government
+- benefit from a smaller CSS payload
+- be able to customise the page template further than before
+
+## Updating to v6
+
+Read the [GOV.UK Frontend v6.0.0 release notes](https://github.com/alphagov/govuk-frontend/releases/v6.0.0).
+
+There are actions you must make to migrate to v6.0.0. These are items you need to update, check or remove.
+
+You might need to update some items, including:
+
+- your Sass compiler to [Dart Sass v1.79.0 or newer](https://sass-lang.com/dart-sass/)
+- using the new GOV.UK web palette 
+- using the new colour functions, and replacing colours to follow the new naming convention
+
+You'll need to check some items still work as expected, which include:
+
+- your service’s custom colours for contrast and legibility with the new web palette
+- any custom components that reference functional colours or CSS custom properties
+- any custom header or footer content if you now include Service navigation or Phase banner components within the `<header>` element
+- testing the new type scale in your service to see if you need to make any adjustments, such as spacing or positioning values
+
+You'll also need to remove references to any APIs, Sass variables and component options we've that removed.