improvements to docs
in this PR I've renamed gnmiclient to gnmic across the doc articles as well as proofread the command ref and basic usage once again, fixing minor stuff.
bumped mkdocs to 5.3.0 to support dark mode
preview https://rdodin.gitlab.io/gnmiclient-docs/
The fixed index page will be worked last, once the docs for multiple outputs will be added
Edited by Roman Dodin