Fix broken/missing crossref links

Live previews:
http://bitcoindevtest.us.to/en/developer-guide
http://bitcoindevtest.us.to/en/developer-reference

There is still some remaining FIXME links. All other links have been tested. So there's very few work left to do before all of them are fixed.

@harding I didn't know where we should point "certificate chain" so I left it in the FIXMEs.

Once this is merged, we should be careful to check if links need to be updated when moving/removing texts or renaming titles.