Introduces `sphinx-tags` to the documentation system, allowing
individual pages to be tagged for better searching. The examples in this
commit introduce the `chip:*` tag for indicating the chip a board uses,
as well as the `experimental` tag for indicating experimental boards and
features. Other tags indicate supported peripherals for boards, such as
`wifi` and `ethernet`.
Signed-off-by: Matteo Golin <matteo.golin@gmail.com>
4f652e0622