Explain what `is_page` attribute is used for.

This was originally added in 58e266ea9f3d377a8eccbd345385c20179bcd080.
Allow non-blog pages to set page title.

Previously pages which used the standard layout had their page title
hard-coded. I've changed the standard layout to use the value of the
`page_title` attribute.

Previously the home page `page_title` attribute value was being ignored,
so in order to preserver the original page title for the home page I've
changed the value of the `page_title` attribute.

In case there are any other pages using the standard layout which do not
have a `page_title` attribute value set, I've set the default to be the
same as the original hard-coded value.

So hopefully this commit does not alter any behaviour, but it does give
us the possibility of setting the page title for new non-blog pages
which we create.
Add a basic page for the Show and Tell events.

Note that I had to use HTML for the bullet points, because I needed
to generate paragraph tags inside the list item tags to avoid the
bullet points looking tiny due to the CSS styles for non-blog pages
not being setup very well.

Even so the page doesn't look great, but hopefully it'll do for now.

Note that I set the `is_page` attribute so that this page is included
in the sitemap. Hopefully since my previous commit, the `page_title`
attribute value will also be honoured.