Update PEtab extensions

[dweindl]

Rephrase and rearrange some extension-related parts:

• Trying to make the PEtab 2.0 specs more self-contained
• Separate process and specs for PEtab extensions
• Avoid using "extension" to refer to different things

[matthiaskoenig]

This looks good. Do we have any extensions which are currently used? If yes it makes sense to list them on the webpage.

[dweindl]

Do we have any extensions which are currently used? If yes it makes sense to list them on the webpage.

There's this:

PEtab/README.md

Lines 112 to 128 in 26c4fb2

	## PEtab extensions
	Other standardization-related projects that extend or build on PEtab include:
	* [PEtab Select](https://petab-select.readthedocs.io/en/latest/)
	PEtab Select brings model selection to PEtab. It comprises both a
	standardized way to define model selection problems in PEtab and a Python
	library to work with these problems.
	* [petabunit](https://github.com/matthiaskoenig/petabunit) (WIP)
	A Python library that extends PEtab to annotate and convert physical units.
	* [PEtab SciML](https://github.com/PEtab-dev/petab_sciml) (WIP)
	PEtab SciML brings Scientific Machine Learning (SciML) models that
	combine mechanistic and machine learning models to PEtab. It comprises
	both a standardized way to define SciML problems, and a Python library
	to work with these problems.

[dilpath]

Suggested change

	While such extra column may, for example, contain metadata to enhance plotting,
	While such extra columns may, for example, contain metadata to enhance plotting,

[dilpath]

Does this contradict the ability of extensions to alter the definition through new columns?

Or does "non-standard" include both "core" and "extension" columns?