Dart glossary: links to other guide chapters go to TS docs

[chalin]

While the broken links in the glossary (#1123) have been fixed, the fix only works for TS because the URLs in glossary.jade are hardcoded to the TS docs.
#1836 implements a redirect for the old "one level up from guide" glossary page. Once this is done, the glossary.jade file links can be made relative (which will fix this issue).

/cc @kwalrath @wardbell @naomiblack

[chalin]

Closed by #2100