Use Sphinx Builder method to get page url

[attakei]

Overview

This PR is to generate og:url correctly for other dirhtml-based builders.

Details

I develop sphinx-revealjs (Sphinx extension to generate presentation) that have dirrevealjs builder extended dirhtml.
I am adding opengraph metatags for dirrevealjs outputs using sphinxext-opengraph, but it does not work for expected.

This has generated /index.html for og:url when use dirrevealjs builder.
But, this should generate / ended.
(For reproduction, please see my reproduction demo.)

For fix(about diff)

Currently, get_tags build page_url manually for dirhtml.
Sphinx builder class has get_target_uri for generate document url from pagename to ref page.
I hope to use this to build og:url.

[TheTripleV]

Thanks!