Update CoreNEURON documentation (#950)

* Update CoreNEURON documentation
 * add detailed instructions for building and using CoreNEURON
 * Add example references

* Add bbcorepointer documentation

* Change url of ringtest and minor updates

* Update bbcorepointer.md

* Update coreneuron.md

* Update coreneuron.md

* Apply suggestions from code review

Co-authored-by: Alexandru Săvulescu <[email protected]>

Co-authored-by: nrnhines <[email protected]>
Co-authored-by: Alexandru Săvulescu <[email protected]>

docs/coreneuron/how-to/bbcorepointer.md

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+
+## Transferring dynamically allocated data between NEURON and CoreNEURON
+
+
+User-allocated data can be managed in NMODL using the `POINTER` type. It allows the
+programmer to reference data that has been allocated in HOC or in VERBATIM blocks. This
+allows for more advanced data-structures that are not natively supported in NMODL.
+
+Since NEURON itself has no knowledge of the layout and size of this data it cannot
+transfer `POINTER` data automatically to CoreNEURON. Furtheremore, in many cases there
+is no need to transfer the data between the two instances. In some cases, however, the
+programmer would like to transfer certain user-defined data into CoreNEURON. The most
+prominent example are random123 RNG stream parameters used in synapse mechanisms. To
+support this use-case the `BBCOREPOINTER` type was introduced. Variables that are declared as
+`BBCOREPOINTER` behave exactly the same as `POINTER` but are additionally taken into account
+when NEURON is serializing mechanism data (for file writing or direct-memory transfer).
+For NEURON to be able to write (and indeed CoreNEURON to be able to read) `BBCOREPOINTER`
+data, the programmer has to additionally provide two C functions that are called as part
+of the serialization/deserialization.
+
+```
+static void bbcore_write(double* x, int* d, int* d_offset, int* x_offset, _threadargsproto_);
+
+static void bbcore_read(double* x, int* d, int* d_offset, int* x_offset, _threadargsproto_);
+```
+
+The implementation of `bbcore_write` and `bbcore_read` determines the serialization and
+deserialization of the per-instance mechanism data referenced through the various
+`BBCOREPOINTER`s.
+
+NEURON will call `bbcore_write` twice per mechanism instance. In a first sweep, the call is used to
+determine the required memory to be allocated on the serialization arrays. In the second sweep the
+call is used to fill in the data per mechanism instance.
+
+The functions take following arguments
+
+* `x`: A `double` type array that will be allocated by NEURON to fill with real-valued data. In the
+  first call, `x` is NULL as it has not been allocated yet.
+* `d`: An `int` type array that will be allocated by NEURON to fill with integer-valued data. In the
+  first call, `d` is NULL as it has not been allocated yet.
+* `x_offset`: The offset in `x` at which the mechanism instance should write its real-valued
+  `BBCOREPOINTER` data. In the first call this is an output argument that is expected to be updated
+  by the per-instance size to be allocated.
+* `d_offset`: The offset in `d` at which the mechanism instance should write its integer-valued
+  `BBCOREPOINTER` data. In the first call this is an output argument that is expected to be updated
+  by the per-instance size to be allocated.
+* `_threadargsproto_`: a macro placeholder for NEURON/CoreNEURON data-structure parameters. They
+  are typically only used through generated defines and not by the programmer. The macro is defined
+  as follows:
+
+```
+#define _threadargsproto_                                                                         \
+    int _iml, int _cntml_padded, double *_p, Datum *_ppvar, ThreadDatum *_thread, NrnThread *_nt, \
+    double _v
+```
+
+Putting all of this together, the following is a minimal MOD using BBCOREPOINTER:
+
+```
+TITLE A BBCOREPOINTER Example 
+
+NEURON {
+    BBCOREPOINTER my_data
+}
+
+ASSIGNED {
+    my_data
+}
+
+: Do something interesting with my_data ...
+
+VERBATIM
+static void bbcore_write(double* x, int* d, int* x_offset, int* d_offset, _threadargsproto_) {
+    if (x) {
+        double* x_i = x + *x_offset;
+        x_i[0] = _p_my_data[0];
+        x_i[1] = _p_my_data[1];
+    }
+    *x_offset += 2; // reserve 2 doubles on serialization buffer x
+}
+
+static void bbcore_read(double* x, int* d, int* x_offset, int* d_offset, _threadargsproto_) {
+    assert(!_p_my_data);
+    double* x_i = x + *x_offset;
+    // my_data needs to be allocated somehow
+    _p_my_data = (double*)malloc(sizeof(double)*2); 
+    _p_my_data[0] = x_i[0];
+    _p_my_data[1] = x_i[1];
+    *x_offset += 2;
+}
+ENDVERBATIM
+```
+