doc: adding a preface

Rewrite some parts of The Munin Guide.

The inspiration is *clearly* PostgreSQL's Book.
doc: fixed typo in preface
doc: build_ can be a symbolic link
doc: adding some preface

   * history
   * about
   * convention
   * bug report
doc: adding bug report section
doc: adding "getting help" section
doc: remove non-working search macro
doc: change settings
doc: fix typo in header
doc: adding "Where to Report" section