docs: fix CLI page
docs: fix CLI page

- move plugin-sideloading section up (under the plugin-config section)
- two blank lines for 2nd layer headlines, one for 3rd layer headlines
- fix (most) punctuation and grammar errors
- fix outdated description in proxy-support section

bastimeyer/streamlink,streamlink/streamlink,chhe/streamlink,bastimeyer/streamlink,gravyboat/streamlink,chhe/streamlink,gravyboat/streamlink,streamlink/streamlink,melmorabity/streamlink,melmorabity/streamlink