Clean up description via HTML escape

[markpeek]

As the description is taken from the raw text of the document it can contain some HTML reserved characters. This change will escape the description string to use HTML entities instead.

This was causing an error when generating a sphinx EPUB document.

[TheTripleV]

I wonder what users of og:desc do. Like how slack and discord embeds display the escaped version. It should be fine but just wanna check.

[markpeek]

Thanks @TheTripleV for looking into the PR. I created a quick reproduction of the issue via this repo in case you wanted to look at it yourself.

[methane]

I want to remove all opengraph meta tag from ePub.
Would you check if app.builder.embedded: return in html_page_context?