Improve pagefind meta

[yueim]

Description

Improved the pagefind metadata.

Types of changes

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Documentation Update (if none of the other choices apply)
• Others (any other types not listed above)

1. Fall back to social image (og:image) if no image is found on the page.
2. Using post content as data-pagefind-body should be better.

Checklist

• I have read the Contributing Guide
• I have added the necessary documentation (if appropriate)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)

Further comments

Related Issue

Closes: #

[yueim]

Read Setting up metadata for more details.