docs: add command reference

currently this is not automatically updated after a change in remage.
RTD does not have a compiled version of remage available, so this can
also not be easily added.

To manually recreate the command docs, one can use:
```
cd build
make remage-doc-dump
./src/remage-doc-dump doc
cd ../docs
./g4manual2rst.py ../build/man/rmg-manual.txt
```

docs/g4manual2rst.py

@@ -0,0 +1,69 @@
+#!/bin/python
+
+"""Convert an output file of remage-doc-dump to a rst file."""
+
+import math
+import re
+import sys
+
+if len(sys.argv) < 2:
+    msg = "need to pass an original file"
+    raise ValueError(msg)
+
+path = sys.argv[1]
+
+outlines = ["remage macro command reference", "=" * 31, ""]
+infile = open(path, "rt")
+inlines = [line.strip("\n") for line in infile]
+
+
+def remove_whitespace_lines_end(lines: list):
+    for i in range(len(lines) - 1, 0, -1):
+        if lines[i].strip() not in ("", "::"):
+            break
+        del lines[i]
+
+
+idx = 0
+in_cmdblock = False
+lastlevel = -1
+
+for line in inlines:
+    if re.match(r"Command directory path : /RMG/", line):
+        remove_whitespace_lines_end(outlines)
+        outlines.extend(["", line, "-" * len(line), ""])
+        in_cmdblock = True
+        lastlevel = -1
+    elif re.match(r"Command /RMG/", line):
+        remove_whitespace_lines_end(outlines)
+        outlines.extend(["", line, "^" * len(line), ""])
+        in_cmdblock = True
+        lastlevel = -1
+    elif in_cmdblock and (line == "Guidance :"):
+        pass
+    elif in_cmdblock and (inlines[idx - 1] == "Guidance :") and not line.startswith(" " * 2):
+        outlines.extend([line, ""])
+    elif in_cmdblock and line == " Commands : " and not inlines[idx + 1].startswith(" " * 4):
+        pass
+    elif in_cmdblock and line != "":
+        stripped_line = line.lstrip()
+        indent = math.ceil((len(line) - len(stripped_line)) / 2)
+        if indent > lastlevel + 1:  # parts of the output have the wrong indentation.
+            indent = lastlevel + 1
+        m = re.match(r"(.*)( [:* ] ?)(.*)?$", line)
+        if m:
+            g = list(m.groups())
+            sep = g[1].strip()
+            fmt = "**" if sep == ":" else "*"
+            if len(g) > 1:
+                g[0] = f"{fmt}{g[0].strip()}{fmt}"
+                g[1] = ": "
+            if len(g) > 2 and g[2] != "":
+                g[2] = f"``{g[2].strip()}``"
+            stripped_line = "".join(g)
+        outlines.append("    " * indent + "* " + stripped_line)
+        lastlevel = indent
+    idx += 1
+
+outfile = open("rmg-commands.rst", "wt")
+outfile.writelines([l + "\n" for l in outlines])

docs/index.rst

@@ -104,6 +104,7 @@ Next steps
     :maxdepth: 2
 
     api/index
+    Geant4 command interface <rmg-commands>
     Good ol' Doxygen <https://remage.readthedocs.io/en/latest/doxygen/annotated.html>
 
 .. |remage| replace:: *remage*