Change linkchecker in tox.ini to run against docs dir (#4093)

Change linkchecker to run against docs dir to validate only links
provided by user instead of all auto-generated links from site dir. All
auto-generated links in site directory contains absolute links. The
problem is that linkchecker tries to traverse those links and is getting
404 error, which is expected as pages are not yet published. This blocks
pull-request for adding new pages or restructuring existing docsite as
in this pr[1].

[1]#4044

Co-authored-by: mciecier <[email protected]>

tox.ini

@@ -86,8 +86,8 @@ description = Build documentation
 passenv = *
 usedevelop = true
 commands =
+    linkchecker -f linkcheckerrc docs
     mkdocs build --strict
-    linkchecker -f linkcheckerrc site
 extras =
     docs