Address SEO Changes on Blog Page

[OughtPuts]

This PR adds the changes outlined in this task in the SEO project to the blog index page.

In summary, it:

• Adds an h1 and introductory paragraph to the page
• Converts the tab headers to h2 elements
• Adjusts the html structure of the blog posts so that the dates are now p tags (not h6), and the dl, dt and dd elements are replaced with a ul/li structure.

Additionally it:

• Converts the introductory paragraphs on other pages (world, community and foundation) to p elements from h4s for consistency.

NB while the elements themselves have changed, the styling has been maintained through CSS changes.

Before:

After: