docs: discord badge with join link (#2473)

* add discord link

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* Add Discord link to social section in mkdocs.yml

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

* Add Discord link to getting started documentation

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

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Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>
Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>
Co-authored-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>