doc: reworked install doc

    * Encourage packaged use
    * Tell more about the architecture
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
/node.d/tomcat_threads.in: Fix if {'currentThreadsBusy'} go false when it is 0, and add a total value as 'currentThreadCount' so that the chart looks more readable.
Add a lower limit of 0 to postgres database size

This ensures that the graph always starts at 0 in stead of somewhere near the
smallest database size. This makes sure that database sizes are shown
proportionally correct.
Make the postgres_size_ y-axis behaviour configurable.
doc: adding page about plugin env
doc: fix table for env.rst
doc: adding the url to the PgSQL manual
doc: using cross doc ref
doc: add the tutorial skel
doc: adding a starting blurb