Edited and reworked content in the Extension Guides Overview and the Generator and Main Extension guides.
Signed-off-by: Paul Williams <pawilliams@mirantis.com>

Update docs/extensions/guides/main-extension.md
Co-authored-by: Panu Horsmalahti <panu.horsmalahti@iki.fi>

Update docs/extensions/guides/generator.md
Co-authored-by: chh <1474479+chenhunghan@users.noreply.github.com>
Signed-off-by: Paul Williams <pawilliams@mirantis.com>
Co-authored-by: Paul Williams <pawilliams@mirantis.com>
Co-authored-by: chh <1474479+chenhunghan@users.noreply.github.com>