Replace manual crossref links by automatic crossref links

Live preview: 
http://bitcoindevtest.us.to/en/developer-guide
http://bitcoindevtest.us.to/en/developer-reference

Diff for the final generated HTML files:
http://bitcoindevtest.us.to/diff.html

Except for explicit links to pages (e.g. ..as seen in the "Transaction" section..), I've dropped all manual crossref links and made sure they were either generated automatically, or more or less required (e.g. near equivalent crossref links). This way links are handled consistently and texts are more readable.

In the absence of critical feedback, this pull request will be merged on May 5th.