update onboarding docs

[RayBB]

Ironically, no associated ticket. I'm just trying to improve our onboarding docs before adding anything about remote development (gitpods / github codespaces).

The improvements are generally:

• Putting linebreaks in long lines. (this has no visual changes)
• Wordsmithing - making descriptions shorter and hopefully clearer
• Removing links that aren't useful
• Add the docker command to the main readme

I try to err on the side of being as clear as possible for new users.

If you think anything should be phrased differently feel free to edit the pr directly, make a comment, or make a PR.

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@jamesachamp

[jimchamp]

I think that this will be good to go once the colon is replaced by a period in the Code of Conduct section.

The Installation section is probably fine. I'm assuming that folks will read the Docker readme if they encounter any issues.

[jimchamp]

Lgtm. Thanks for all of the work that you're doing to improve Open Library's documentation!