Remove Planet FreeBSD from the handbook and website

[carlavilla]

Summary

Sadly, planet.freebsd.org don't works anymore. This patch remove the links from the website and from the handbook.

Fix that report: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=211706

Test Plan

Compile the website and the handbook.

[bcr]

Approved. Can you commit that change yourself or should I?
Thanks for picking this up.

[carlavilla]

In D22221#485618, @bcr wrote:

Approved. Can you commit that change yourself or should I?
Thanks for picking this up.

No, I can't, I only have access to es_ES.
Please, make the commit yourself. When the commit is done, I'll close the ticket in bugzilla.

Thanks for the review!