docs(website): explain all property domains

[danielelotito]

Not all fields in https://github.com/danielelotito/ollang_1rst_try/blob/master/.venv/lib/python3.12/site-packages/orchestrator/schema/domain.py#L13 are explained in the docs.
For instance, searching for OPEN_CATEGORICAL_VARIABLE_TYPE in the website leads to no match.
Done when:

• Custom experiment explains this possibility (https://ibm.github.io/ado/actuators/creating-custom-experiments/)
• References to unbounded variables are connected to OPEN types (https://ibm.github.io/ado/core-concepts/entity-spaces/#entity-spaces)

Nice to have

• We have a dedicated page in the website that go through the possibilities in https://github.com/danielelotito/ollang_1rst_try/blob/master/.venv/lib/python3.12/site-packages/orchestrator/schema/domain.py#L13 and we properly connect this page to custom experiments and entity space docs.