docs: uniffi = { workspace = true } does not work

[jo]

When following the instructions on how to set up a new component, its advised to declare uniffi as a workspace dependency

[dependencies]
uniffi = { workspace = true }

but running cargo check -p results in an error:

error: failed to load manifest for workspace member `/home/jo/moz/application-services/components/init_rust_components`
referenced by workspace at `/home/jo/moz/application-services/Cargo.toml`

Caused by:
  failed to parse manifest at `/home/jo/moz/application-services/components/init_rust_components/Cargo.toml`

Caused by:
  error inheriting `uniffi` from workspace root manifest's `workspace.dependencies.uniffi`

Caused by:
  `workspace.dependencies` was not defined

┆Issue is synchronized with this Jira Task
┆Epic: Important backlog

[mhammond]

oops - we did that for a while, but then struck issues after vendoring the change to m-c, so undid it.

iow, the docs need updating.

[jo]

#6626