Update docs frontpage to show mapping metadata and examples at a glan…

…ce (#393)

Resolves #392 

This PR updates the front page to show at a glance the mapping level
metadata fields, and a better view of a SSSOM example table.

- [X] `docs/` have been added/updated if necessary
- [ ] `make test` has been run locally
- [ ] tests have been added/updated (if applicable)
- [ ]
[CHANGELOG.md](https://github.com/mapping-commons/sssom/blob/master/CHANGELOG.md)
has been updated.

Makefile

@@ -89,6 +89,7 @@ $(DOCDIR):
 
 gendoc: $(DOCDIR)
 	cp -rf $(SRC)/docs/* $(DOCDIR) ; \
+	$(RUN) jinjanate $(SRC)/doc-templates/frontpage.md.jinja2 $(SOURCE_SCHEMA_PATH) -o $(DOCDIR)/index.md
 	$(RUN) gen-doc -d $(DOCDIR) $(SOURCE_SCHEMA_PATH) --template-directory $(TEMPLATE_DIR) --index-name linkml-index
 
 testdoc: gendoc serve