libcamera: Add missing SPDX headers

Multiple files in libcamera are missing SPDX headers. Add them with the
following licenses:

- CC-BY-SA-4.0 for documentation

- CC0-1.0 for build system, development tool configuration files and
  TODO lists, as we consider them non-copyrightable, and for example
  configuration files to facilitate their use

- BSD-2-Clause for a file copied from the Raspberry Pi tuning tool (and
  add the corresponding copyright information)

While at it, add a missing blank line to
src/libcamera/pipeline/virtual/README.md.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>

Documentation/mainpage.dox

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC-BY-SA-4.0
+
 /**
 \if internal
 \mainpage libcamera internal API reference

Documentation/thread-safety.dox

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC-BY-SA-4.0
+
 /**
  * \page thread-safety Reentrancy and Thread-Safety
  *