docs: rename all .txt files to .asciidoc
docs: update references inside manuals

manual links were updated to the proper `.asciidoc` references
documenting manual was made as a concept for starting with updating
the documenting process see machinekit/machinekit#100
docs: update link typo error
docs: fix link to contributing machinekit.io site
docs: add table with navigable link back to the Documents index
Merge commit '23eb65dfbdd2dea6ee1d8b17048042ba9c218d91' into subtree-docs