Rename tech notes to files with reasonable names.

Keep the old files to avoid breaking existing links to them, but just point to
the files in the new contributing subdirectory with more meaningful names now.

Also remove a couple of long obsolete tech notes.
Convert contributors documentation to markdown.

The conversion is very superficial and there are probably many problems in the
Markdown rendering, but this is the first step towards making these documents
more readable when viewing them online.