Docs EPUB: fix meta tags connected with Open Graph

[m-aciek]

• Move meta theme-color tag from sphinxext-opengraph custom tags config directly to layout.html. Previously it would be added for all HTML-related Sphinx builders, after it's only added when builder == HTML.
• Partially revert Docs HTML: Remove self-closing tags #132220, bring back self-closing tags in custom meta tags in sphinxext-opengraph config. (I've opened Custom tags and EPUB builder sphinx-doc/sphinxext-opengraph#136 to discuss alternative fixes.)

It's a follow-up to #132220 and #133720. This change should fix EPUB builds. When testing locally for #133720 I didn't have sphinxext-opengraph installed in the environment, that's why I didn't catch it earlier, I'm sorry.

Could we please backport it for supported versions and 3.12?

Before:

After:

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📚 Documentation preview 📚: https://cpython-previews--134071.org.readthedocs.build/

[hugovk]

Do meta tags even make sense for EPUB (and other builders)? If not, it's probably better to skip them altogether for those builders. Could save us future headaches as well.

[m-aciek]

Do meta tags even make sense for EPUB (and other builders)? If not, it's probably better to skip them altogether for those builders. Could save us future headaches as well.

It looks like they don't. https://chatgpt.com/share/6827445c-70f0-8004-bed3-353546e96afa

sphinx-doc/sphinxext-opengraph#137 @methane started the implementation of skipping them on the extension side.

[hugovk]

It looks like they don't. chatgpt.com/share/6827445c-70f0-8004-bed3-353546e96afa

I'd prefer an authoritative source rather than a webpage that says "ChatGPT can make mistakes. Check important info."

[m-aciek]

It looks like they don't. chatgpt.com/share/6827445c-70f0-8004-bed3-353546e96afa

I'd prefer an authoritative source rather than a webpage that says "ChatGPT can make mistakes. Check important info."

I wasn't able to confirm that searching manually in a limited time.