Update software citation handling and add citation markdown for tsdate

Author: hossam26644

_includes/software-citation.html

@@ -7,7 +7,7 @@
         {% remote_include app.citation_md %}
       {% endcapture %}
       <div style="margin-top: 1em; font-size: 13px;">
-        {{ citation_md | replace: "(sec_citation)=", "" | replace: "(citation)=", "" | markdownify }}
+        {{ citation_md | split: "(sec_citation)=" | last | markdownify }}
       </div>
     {% endif %}
 

_software/tsdate.md

@@ -14,5 +14,6 @@ code_snippet: |2
 redirect_from:
   - /tsdate/
 citation_url: https://tskit.dev/tsdate/docs/stable/#citing
+citation_md: https://raw.githubusercontent.com/tskit-dev/tsdate/refs/heads/main/docs/citation.md
 ---
 Tsdate is a scalable method for estimating the age of ancestral nodes in a tree sequence.