docs: Make giscus work correctly on GitHub hosted docs

This change fixes giscus for https://hyperledger-labs.github.io/splice
by hard-coding documentation page pathname (which is used as a
discussion title by giscus) instead of relying on location.pathname from
the browser which is prefixed with `<reponame>/` when hosted on GitHub.

Signed-off-by: Stanislav German-Evtushenko <[email protected]>

Author: giner

docs/src/_templates/page.html

@@ -8,7 +8,8 @@ <h2>Comments<a class="headerlink" href="#comments"></a></h2>
     data-repo-id="R_kgDONbkPcQ"
     data-category="comments-for-docs"
     data-category-id="DIC_kwDONbkPcc4Cnaia"
-    data-mapping="pathname"
+    data-mapping="specific"
+    data-term="{{ pagename }}"
     data-strict="1"
     data-reactions-enabled="0"
     data-emit-metadata="0"