Update doc for development environment setup

[jobselko]

This PR updates the quickstart documentation to make the development environment setup clearer and replaces the relevant section in README with a link to this file.

It also fixes the misspelled variable _pdbreset_help in the install_phelpers.sh file, which caused that an empty string was used instead of the variable's content.

[pedro-psb]

Cool, it's useful to make a clear distinction on required vs optional.

Since you are editing this, I think its worth mentioning p commands (see oci_env phelp).

When Gerrod introduced them to oci_env some months ago I found them to be really handy on day-to-day debugging to reset state (workers or db) without restarting the container.

Its just an idea, if you don't feel like doing it here I can add that later (just though it was worth mentioning).

[jobselko]

Thank you, any suggestions are always welcome! I added a note regarding "p commands". Please check if it is accurate.

[pedro-psb]

That's a nice place and size to put it. Thanks!

AFAIK, these commands handle s6 services, which are multiple processes within the (single) dev container.

I guess it's more accurate to say something like:

- It contains "p commands" to ease managing pulp component services, such as workers (api, content and tasking) and postgres DB.