exporters/markdown_multipage: use .md extension for inter-page links …

…when possible

- inter-page links must only use .html extension when followed by an anchor - e.g. ../index.html#list-of-licenses
- workaround for executablebooks/MyST-Parser#564, executablebooks/MyST-Parser#749

hecat/exporters/markdown_multipage.py

@@ -186,7 +186,7 @@
 
 ## Software
 
-This page lists all projects in this category. Use the [index of all projects](../index.html), the sidebar, or click on {octicon}`tag;0.8em;octicon` tags to browse other categories.
+This page lists all projects in this category. Use the [index of all projects](../index.md), the sidebar, or click on {octicon}`tag;0.8em;octicon` tags to browse other categories.
 
 """