doc: add CrOS install guide (#225)

* doc: add CrOS install guide

Signed-off-by: Hikari <enra@sayonika.moe>

* doc: change occurences of index to install guide

Signed-off-by: Hikari <enra@sayonika.moe>

* doc: add penguin.linux.test as alternative endpoint

Signed-off-by: Hikari <enra@sayonika.moe>

* doc: link Crostini and crouton info pages and describe install guide

Signed-off-by: Hikari <enra@sayonika.moe>

* doc: remove citations for dev mode requirement

Signed-off-by: Hikari <enra@sayonika.moe>

* doc: clarify more wording

Signed-off-by: Hikari <enra@sayonika.moe>

* doc: fix typo in Crostini section

Signed-off-by: Hikari <enra@sayonika.moe>