Standardize documentation and make it more uniform

[remyroy]

We have documentation in various places that use different terms, vocabulary, objectives and carry different intentions. Here is a short list:

• The README file: https://github.com/eth-educators/ethstaker-deposit-cli/blob/main/README.md
• The click documentation when calling from the CLI (in the source code)
• Source code comments
• mdBook published on https://eth-educators.github.io/ethstaker-deposit-cli/ and sourced from https://github.com/eth-educators/ethstaker-deposit-cli/tree/main/docs

We should do a pass to standardize everything based on the latest efforts on the mdBook from https://eth-educators.github.io/ethstaker-deposit-cli/ .

[remyroy]

All that is left here would be to make the documentation in our source code and the one for the click CLI help the same as the one we can find in the documentation on https://deposit-cli.ethstaker.cc/ where it applies and taking the work, expressions and phrasing used on https://deposit-cli.ethstaker.cc/ as the reference point.