Add a postMainContent block to the base page template

[helenb]

See Link to Jira issue

• When working on the CMS PR I found another place where it would be beneficial to have a block to override - at the moment we have to include the backToTop link on every page, and it would be easier to use a block in the base template. So I have added a new block after the mainContent called postMainContent.
• Updated the 'template block areas' example and relevant VR tests
• Updated the base template snapshot
• Updated the migration instructions

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Check code changes and updated example template, and ensure CI checks pass.

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Project	Deployment	Actions	Updated (UTC)
design-system	Ready	Preview, Comment	Jul 29, 2026 4:19pm