Update docs section in CONTRIBUTING
Update docs section in CONTRIBUTING

Link to the site for the live documentation instead of the wiki.
Remove indications to update the ZAP wikis, external contributions are
no longer allowed and the wikis are being phased out.

Signed-off-by: thc202 <43af82ef30812a20fbcf38b99a6f72cdd3a89ac1@gmail.com>

psiinon/zaproxy,kingthorin/zaproxy,thc202/zaproxy,thc202/zaproxy,zaproxy/zaproxy,psiinon/zaproxy,psiinon/zaproxy,thc202/zaproxy,psiinon/zaproxy,thc202/zaproxy,zaproxy/zaproxy,kingthorin/zaproxy,zaproxy/zaproxy,kingthorin/zaproxy,zaproxy/zaproxy,psiinon/zaproxy,thc202/zaproxy,zaproxy/zaproxy,kingthorin/zaproxy,kingthorin/zaproxy,kingthorin/zaproxy,thc202/zaproxy,zaproxy/zaproxy,psiinon/zaproxy,kingthorin/zaproxy,psiinon/zaproxy,zaproxy/zaproxy,thc202/zaproxy