add `title_html` field to doc and render code blocks

[LeoMcA]

content would like to add code blocks to titles using markdown backtick code syntax.

With a title like `<input>` HTML element the intended result would be:

In HTML contexts, to render the backticks out to HTML <code> blocks, e.g:

<h1><code>&gt;input&lt;</code> HTML element</h1>

In non-HTML contexts, to drop the backticks entirely, e.g.

<meta name="og:title" content="&lt;input&gt; HTML element" />

There may be existing logic to handle this sort of thing already, since we have mixed <code> element and text nodes in some sidebar titles.

It'll be the safest backwards-compatible change to introduce a new title_html field to render the <code> blocks in, and continue to use title (and pageTitle, and any other title-related fields we have...) for the plaintext no-backtick version.

We should also ensure there's an escaping mechanism, or we handle titles with no "closing" backtick, as there are already some titles including the character.