docs: Extends #999. Remove disused package readmes. Rename to sr-metadata. Fix Readme Rustdoc names (#1016)

* docs: Remove safe-mix since is no longer local dependency

* docs: Fix existing packages link. Still only includes Substrate Core

* docs: Remove empty package Readmes. Remove old docs links. Rename to Readme

* docs: Remove link to old docs

* misc: Merge latest from master. Renaem to sr-metadata. Fix list of RustDoc package names

* Rename substrate-metadata to sr-metadata, since it is in the "srml" subdirectory

* Change example to use package name that opens all packages in RustDocs

* Fix list of packages names that are available to open in RustDocs

* fix typo

* fix ordering of CLI options and add missing backslash