Documentation/technical: include Scalar technical doc

Documentation/technical: include Scalar technical doc

Include 'Documentation/technical/scalar.txt' alongside the other HTML
technical docs when installing them.

Now that the document is intended as a widely-accessible reference, remove
the internal work-in-progress roadmap from the document. Those details
should no longer be needed to guide Scalar's development and, if they were
left, they could fall out-of-date and be misleading to readers.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>