create base documentation (issue #59)

This version introduces multiple enhancements:

added some base structure for how to start
created some new documentation files (language-usage and url-conventions)

Details of additions/deletions below:
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Added:      /docs/language-usage.md
            -- Added --
                documentation with using translations

Added:      /docs/url-conventions.md
            -- Added --
                documentation with mode rewrite and classic url conventions

Modified:   /docs/how-to-start.md
            -- Added --
                some base documentation structure