Feature idea: front-end keyword / structure checker

SEO is often equivalent to alchemy - there is no exact science or no one standard to follow. However, tools like Yoast have a robust set of checks to ensure the basics.

We should be providing a simple visual check of basic metrics on the page, such as:
* Check that correct meta elements are present.
* Check that meta contents are of an appropriate length.
* Check the heading and paragraph structure.
* Word counts / estimated read time of page.
* Show a simple Google search preview (visual replication of what it might look like in search results).

These metrics can be grouped by who is responsible - for example if meta elements are completely missing, it is a developer problem. If meta elements are present but empty, it is a content editor problem.

Example of Yoast on a WordPress site:

No files, tests, or entry points are named. Start by surveying the repository for its Wagtail integration and front-end entry points, then turn the proposed checks into an agreed scope with examples. Done should be defined by the selected checks, responsibility grouping, and search-result preview behavior all being implemented and tested.

Written by the indexing model from the issue text.