Add intro synchronous clocks

[clagms]

Adds the introductory text described in #1073

[andreas-junghanns]

I like it - brief intro with problem description and example.
Can we build this example in the Reference FMUs as examples?

[andreas-junghanns]

Do you think you want to enhance this further and keep it in draft state?
When you think you are done, please mark as "ready for review".

[clagms]

Can we build this example in the Reference FMUs as examples?

Good idea. This would also validate the explanation. I've created https://github.com/modelica/fmi-standard/issues/1150 to keep track of this.

[andreas-junghanns]

@masoud-najafi , @TorstenBlochwitz : can either of you read through the PR? I would like at least one more person to share the blame ;)

[masoud-najafi]

In 2_2_common_schema.adoc I cannot understand the following phrase, is it related to SE or the TriggeredBy attribute?

If <> = <> and <> = <> the <> tick event can occur only in the model partition that is related to the referenced <>, i.e. the <<outputClock,output clock>> value (active/inactive) can only change in this model partition.

[andreas-junghanns]

In 2_2_common_schema.adoc I cannot understand the following phrase, is it related to SE or the TriggeredBy attribute?

If <> = <> and <> = <> the <> tick event can occur only in the model partition that is related to the referenced <>, i.e. the <<outputClock,output clock>> value (active/inactive) can only change in this model partition.

This sentence is not part of this PR. Can you open another a new issue for this?
If this is the only problem you have with this PR, do you approve?