contributing: link to TSC

Provide a link and contact information to the technical steering
committee.

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

update contribution guidelines

* the seL4 foundation requires DCO process instead of a CLA
* minor document re-org and markdown lint fixes

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

Release docs owned by Data61 under CC-BY-SA-4.0

CC-BY-SA-4.0 is a more appropriate license for documentation than BSD
or GPL.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

CONTRIBUTING: use new docsite url

CONTRIBUTING: add additional kernel history rules

CONTRIBUTING: use markdown refs for url

- this avoids duplicating the url

Fix licenses (the rest)

update seL4.systems links

(`Contributing/` page has been moved)

CONTRIBUTING.md file for github