unused/outdated pages should be cleaned up

On https://www.fieldtriptoolbox.org/pagelist I encountered a few pages whose purpose is not clear to me, or that are outdated, or don't work.

https://www.fieldtriptoolbox.org/search_results/ probably remains from an older interface to google search.

https://www.fieldtriptoolbox.org/index/ is listed but gives an error. Technically that is the home page. Possibly a `redirect_from` can fix this.

https://www.fieldtriptoolbox.org/external_links/ has always been outdated, and is probably not linked to from anywhere. Some of the external links could be added to the https://www.fieldtriptoolbox.org/about/ page.

The https://www.fieldtriptoolbox.org/dataformat/ and subpages https://www.fieldtriptoolbox.org/dataformat/meg/ and https://www.fieldtriptoolbox.org/dataformat/spike/ should be merged with https://www.fieldtriptoolbox.org/faq/preproc/datahandling/dataformat/