Expose a page.content_title attribute

[oprypin]

Similar to title but prioritizes the title from the document itself over the title specified in the nav config.

For themes, this should be preferred within the <title> tag. To apply the preferred behavior but keep compatibility with older versions, you can use:

<title>{{ page.content_title or page.title }}</title>

• Closes Consider changing the precedence of title sources mkdocs/mkdocs#3532