Complete schemas-only anchors

[manuelfuenmayor]

Fixes https://github.com/metanorma/iso-10303/issues/73

In this PR, I have just completed the anchors of the schemas-only docs.
I've confirmed the anchors fulfill with this structure:

== action_schema [[action_schema_funds]]
{% if schema.constants.size > 0 %}[[action_schema.constants]]{% for thing in schema.constants %}[[action_schema.{{thing.id | replace: "\", "-"}}]]{% endfor %}{% endif %}
{% if schema.types.size > 0 %}[[action_schema.types]]{% for thing in schema.types %}[[action_schema.{{thing.id | replace: "\", "-"}}]]
{% if thing.items.size > 0 %}[[action_schema.{{thing.id | replace: "\", "-"}}.items]]{% for item in thing.items %}[[action_schema.{{thing.id | replace: "\", "-"}}.items.{{item.id | replace: "\", "-"}}]]{% endfor %}{% endif %}{% endfor %}{% endif %}
{% if schema.entities.size > 0 %}[[action_schema.entities]]{% for thing in schema.entities %}[[action_schema.{{thing.id | replace: "\", "-"}}]]{% endfor %}{% endif %}

[opoudjis]

I honestly reacted with "what, is that it?" But... ok!

[opoudjis]

@manuelfuenmayor TRThurman had said:

• CONSTANTS and

• INTERFACEs

also need to be rendered.

I see constants; what about interfaces?