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>
version of the Pico SDK in their install instructions. All information
in the board README.txt files are now moved into the RST files to help
with the removal of the old READMEs.
