[IATI/IATI-Extra-Documentation#104] Add reference label
Add datastore page
[IATI/IATI-Extra-Documentation#79] activities-standard->activity-standard
Add percentage attribute change to changelog

IATI/IATI-Standard-SSOT#29
IATI/IATI-Schemas#106
Some information to signpost people 

Signposts people to the datastore
Add 404 page
[IATI/IATI-Extra-Documentation#126] Add condition/@xml:lang to changelog
[#4] Update internal links and add table of contents
[#4] Add formatting to licensing page
[#4] Fix typos