(DOCS-14993): Update WT Docs link to current version (#296)

* (DOCS-14993): Update WT Docs link to current version

* Add 4.2 WT link to extlinks

* test

* fix extlinks

Author: jeff-allen-mongo

conf.py

@@ -103,7 +103,7 @@ def has(self, *args):
     'atlas': ('https://docs.atlas.mongodb.com%s',''),
     'compass': ('https://docs.mongodb.com/compass/current%s', ''),
     'products': ('https://www.mongodb.com/products%s', ''),
-    'wtdocs': ('https://source.wiredtiger.com/mongodb-3.4%s', ''),
+    'wtdocs': ('https://source.wiredtiger.com/mongodb-4.2%s', ''),
     'perl-api': ('https://metacpan.org/pod/MongoDB::%s', ''),
     'node-docs': ('https://mongodb.github.io/node-mongodb-native/3.3/%s', ''),
     'node-api': ('https://mongodb.github.io/node-mongodb-native/3.3/api/%s', ''),