Documentation: `tezos-node` requires Zcash parameters

New user onboarding documentation at: https://tezos.gitlab.io/introduction/howtouse.html#node

Describes the first steps of running tezos-node as "First, we need to generate a new identity for the node to connect to the network: tezos-node identity generate".

However, a fresh installation and running tezos-node actually yields the error:

Failed to initialize Zcash parameters: cannot find Zcash params in any of:
- /home/ubuntu/.local/share/zcash-params
- /home/ubuntu/.zcash-params
- /home/ubuntu/_opam/share/zcash-params
- /usr/local/share/zcash-params
- /usr/share/zcash-params
- /var/lib/snapd/desktop/zcash-params
You may download them using https://raw.githubusercontent.com/zcash/zcash/master/zcutil/fetch-params.sh

Speaking frankly this looks sketchy as hell, and it's not clear to a novice Tezos user why one would want to run an unknown script from a totally different cryptocurrent project.

Doing some digging, apparently 1 year ago some Zcash privacy features and something called "Sapling" were integrated into Tezos, but this absolutely must be documented. Subsequently, I also found a repository that looks like it ALSO has Sapling parameters (https://gitlab.com/tezos/tezos-sapling-parameters), so now I am even more unclear as to whether I should install something from Zcash repos, or instead parameters from Tezos' own Gitlab.

Apparently related to #1036