Add fbpostlikes to sponsor section

[wing328]

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Summary by cubic

Added FBPostLikes as a bronze sponsor, showing their logo and link on the website and in the README.
Includes a new image asset (fbpostlikes.png) and a sponsors.yml entry.

^{Written for commit 9812125. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files