Change url & title of ancestry pages

[ljwh2]

The ancestry page: https://www.ebi.ac.uk/gwas/ancestry url should be changed to https://www.ebi.ac.uk/gwas/populationdescriptors Title to be changed to "Population descriptors" on this page and on the homepage

The documentation page https://www.ebi.ac.uk/gwas/docs/ancestry should also be changed to https://www.ebi.ac.uk/gwas/docs/populationdescriptors

[ljwh2]

Note from Yomi: but what if someone has previously bookmarked the previous url or also search index has stored it for Google and … if we care about that, we can just point the 2 URLs to thesame page so we have a new one, and we preserve the previous one as well