[Docs] Quickstart - add pyproject.toml info to the declarative config section

[VladimirFokow]

Summary of changes

Preview.

• Update the section about transitioning from setup.py to declarative config - so that it not only has setup.cfg but also pyproject.toml, and gives important context to the new users who are learning about these files for the first time.

• Extract useful information about these files from GitHub comments to the docs. The new description conforms to the comments:

  • quote

  the approach setuptools would like to take is to eventually use a single declarative format (pyproject.toml) instead of maintaining 2 (pyproject.toml / setup.cfg).

  • quote

  For the time being there it is not in the horizon of events to drop setup.cfg completely. setup.cfg has not even been deprecated yet.
  Even if we eventually drop the custom parsing, chances are it will still keep working via "transpiling" it to pyproject.toml syntax for a long time, until it is safe to assume everyone moved out of it.

  • quote

  Using pyproject.toml, however, imposes constraints (specially around dynamic) to the build. Some users are not comfortable with these constraints, so I don't think there should be a "black and white" recommendation.

Note:
Maybe an info box was more appropriate than note?
I don't know the rules here or if it even matters, but info's syntax looked complicated so I chose a simple .. note::

Please, suggest edits if this section can be made more accurate.
I think it's important to update this section from what it currently is.

Closes #4200

Pull Request Checklist

• Changes have tests
• News fragment added in [newsfragments/]

[abravalheri]

Thank you very much @VladimirFokow for working on this.

[VladimirFokow]

Can also remove "(usually setup.cfg)" from the setup.cfg guide?:

Setuptools allows using configuration files (usually setup.cfg) to define a package’s metadata and other options that are normally supplied to the setup() function (declarative config).

[abravalheri]

Can also remove "(usually setup.cfg)" from the setup.cfg guide?:

Yes, maybe replace usually with for example?

[abravalheri]

Thank you very much!