Improve heading structure

[jcarstairs-scottlogic]

TODO:

• put up a preview deployment
• rebase onto/merge from ScottLogic/gh-pages and resolve conflicts

Some users, especially those who use screen readers, expect headings to be structured in a certain way, and it may be more difficult to use our blog if we don't do so. For that reason, WebAIM, WAI and the Home Office all advise:

• Exactly one <h1> per page
• The <h1> should describe the purpose of the whole page, just like a <title>
• Don't skip heading levels (e.g. <h2> followed by <h4>)
• Make sure headings describe the content underneath them
• Make sure content is organised under relevant headings

This PR collects a large number of small changes to make our blog conform to these guidelines better. This shouldn't have any impact on the visual appearance of the website. The changes either re-number headings (e.g. <h4> -> <h3>), or insert invisible headings which wouldn't make sense in the visual design but which do provide important, meaningful structure for users of screen readers.