[css-navigation-1] Add workflow to auto-publish to gh-pages.

This adds a workflow to auto-publish the css-route-matching draft to
gh-pages.  It moves it into a subdirectory so that only the spec and not
all the other files get put on the gh-pages branch.

(This is part of importing the history from WICG/declarative-partial-updates.)

Author: dbaron

css-navigation-1/Overview.bs

@@ -7,7 +7,7 @@ Org: w3c
 Level: 1
 Repository: WICG/declarative-route-matching
 !Issue Tracking: <a href="https://github.com/w3c/csswg-drafts/issues/12594">w3c/csswg-drafts#12594</a>
-ED: https://wicg.github.io/declarative-partial-updates/css-route-matching.html
+ED: https://wicg.github.io/declarative-partial-updates/css-route-matching/
 Editor: L. David Baron, Google https://www.google.com/, https://dbaron.org/, w3cid 15393
 Editor: Noam Rosenthal, Google https://www.google.com/, w3cid 121539
 Abstract: This module contains conditional CSS rules for styling based on routes