Streamline terminology clause mixed/systematic order issues

[ronaldtse]

There is a problem in documentation that likely stemmed from implementation:

1. The treatment of terms encoding in "systematic order" (concept / subconcept) should be identical whether it is a vocabulary document or a normal document.
2. There are now two ways of adding description text to a mixed order clause:

== Terms and definitions

[.grouping]
=== Term grouping 1
Paragraph 1

==== Term A
Definition A

==== Term B
Definition B

[.grouping]
=== Term grouping 2
Paragraph 2

==== Term C
Definition C

and

== Terms and definitions

=== Term grouping 1

[.boilerplate]
==== {blank}
The following terms are adopted from ISO 10303-2.

==== Term A
Definition A

==== Term B
Definition B

=== Term grouping 2

[.boilerplate]
==== {blank}
The following terms are adopted from ISO 10303-11.

==== Term C
Definition C

The second method seems unnecessary and in fact rather contrived.

Originally posted by @ronaldtse in https://github.com/metanorma/metanorma.org/issues/733#issuecomment-1980387076

[opoudjis]

The treatment of terms encoding in "systematic order" (concept / subconcept) should be identical whether it is a vocabulary document or a normal document.

The implementation allows it in both places, and not validated against.

Boilerplate markup does seem contrived, agreed. Taking out.