generate docs (and other niceties ..) for nimbus sub-directory

why:
  * easy browsing of prototype docs, allows to follow links with
    a web browser on the local file system
  * some md & png files may contain additional documentation

overview:
  * separate nimbus/makefile, try "make -C nimbus" for instructions
  * running "make -C nimbus docs" will do the job
  * x-ref file in nimbus/docs/theindex.html
  * additional md and png files in nimbus/docs/ex/.. subdirectory

details:
  * a newer nim compiler provides better referencing when available, in
    particular the back link to the indices are not provided by the 1.2.10
    nim compiler (automatically handled by makefile)
  * make patterns are used to update files only when the timestamp changes
  * should provide "discount" markdown generator, otherwise fallback
    to <pre/> encapsulated text file

.gitignore

@@ -1,4 +1,4 @@
-/nimcache
+*~
 
 # Executables shall be put in an ignored build/ directory
 /build
@@ -20,6 +20,15 @@
 *.exe
 *.dll
 
+# Ignore generated C and doc files
+nimcache
+/nimbus/docs
+
+# Ignore test reports
 VMTests.md
+/newBlock[Cc]hainTests.md
+/witnessBuilderBC.md
+/witnessBuilderGST.md
+
 /debug*.json
 /block*.json