fix: consolidate command descriptions, order

[line-o]

• always start with a capital character
• remove redundant information (~instance)
• fix wording (connection -> collection)
• re-order and group commands
• display commands in table with aliases
• use main command in examples instead of aliases

Preview of readme after changes https://github.com/eXist-db/xst/blob/f5ab970b720562c053f3cd68503cc441cb66962b/readme.md

Thanks to valuable feedback from @JoernT and Pieter

[github-actions[bot]]

:tada: This PR is included in version 1.2.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: