create base documentation (issue #59)

This version introduces multiple enhancements:

added documentation of framework configuration
added part of documentation of page structure
removed documentation from tree.dtd

Details of additions/deletions below:
--------------------------------------------------------------

Modified:   /docs/configuration.md
            -- Added --
                added full configuration description

Modified:   /BLUE_FRAMEWORK/cfg/dtd/tree.dtd
            -- Removed --
                removed documentation of elements
Modified:   /docs/page-structure.md
            -- Added --
                part of page structure documentation (tree DTD, base structure, pages, subpages)