MPIDE-25: document high-level concepts and syntax

Added an overview.md to the docs to cover the conceptual and syntactical
part of the language. Gives simple, brief examples.

docs/overview.md

@@ -0,0 +1,53 @@
+# what is modellang?
+`modellang` is an event modeling language built with ease of use and flexibility in mind. The driving concept behind this is the separation of model description into two distinct parts: behaviors and interactions.
+To wit, let us describe a model where one task sends messages and another task will then receieve them:
+```
+SYSTEM: task_a = send*;
+SYSTEM: task_b = receive*;
+
+INTERACTION: task_a:send -> task_b:receive;
+```
+
+`modellang` is not just an ergonomic language, though. Its **trace generator** is able to simulate all possible scenarios of a model within a small scope, producing a queryable set of results.
+
+## language
+A `modellang` model is composed of systems of behaviors, interactions between the systems, and triggers that control properties.
+
+A **system** is a top-level event, and is where trace generation starts. Can consist of composite events (behaviors) or atomic events.
+
+```
+SYSTEM: sys = behavior_pattern1 behavior_pattern2;
+BEHAVIOR: behavior_pattern1 = expanded detail;
+```
+
+An **interaction** can fall into one of three categories: relations. pre-conditions, and post-conditions:
+- the two relations supported are "->" (order) and "==" (join)
+- pre- and post-conditions guarantee your resultant traces abide by certain rules
+
+```
+INTERACTION: sys:before -> other_sys:after;
+INTERACTION: sys:shared_event == other_sys:shared_event;
+INTERACTION: property > 5 -> thing:happens_only_above_5;
+INTERACTION: thing:never_drops_below_three -> other_property >= 3;
+```
+
+A **trigger** is a chunk of code that will execute whenever a specified pattern occurs during trace generation.
+
+```
+WHEN: sys:event1 event2 { property++ };
+```
+
+**Behavior patterns** are the way in which events are described.
+
+```
+SYSTEM: sequence = a whitespace delimited list of events occurring in order;
+SYSTEM: group = you can even (wrap stuff into groups);
+SYSTEM: alternation = a (pipe | vertical bar | stick) delimited list of sequences;
+BEHAVIOR: quantifiers = none_or_more* one_or_more+ none_or_one?;
+BEHAVIOR: ranges = do_exactly_n_times{n} do_at_least_n_times{n,} do_n_to_m{n,m};
+```  
+Notes on patterns:
+- alternations operate on sequences and groups, not individual events, so `a b c | d e | f` would separate into `a b c`, `d e`, and `f`
+
+**Whitespace** is insignificant.  
+**Comments** are C-style.