feature: Show deprecation notices for deprecated functions

[sevenseacat]

Closes #91 .

• Adds a new deprecated field to Function resources (only they can be tagged with @deprecated module attributes)
• Imports the text content of @deprecated module attributes when the importer parses docs for a module
• Displays both in the right nav and in the main body content when functions are deprecated.
• Includes some styling fixes for displaying function names/heads/content (do I like the styling on hexdocs? Yes I do)

Before	After

Also includes headings for extensions docs pages, so they look a bit more presentable.

[zachdaniel]

🚀 Thank you for your contribution! 🚀