Update plugin documentation & refine plugin interface

[eivindjahren]

In https://github.com/equinor/ert/commit/4f363cc0257b658d01c71125118fca6b654a578d & https://github.com/equinor/ert/commit/ceda74fdd18d8df31685288578cfa2b034d839b2 , a new way of handling

import ert

@ert.plugin
def installable_forward_model_steps():
    ...

in such a way that a plugin implementer does not need to import hook_implementation & plugin_response.

While at it, we could create a replacement for legacy_ertscript_workflow so that instead of

@hook_implementation
def legacy_ertscript_workflow(config):
    workflow = config.add_workflow(CsvExport2Job, "CSV_EXPORT2")
    workflow.parser = csv_export_parser
    workflow.description = DESCRIPTION
    workflow.examples = EXAMPLES
    workflow.category = "export"

one would have

@ert.plugin
def ertscript():
    return {
        "ertscript": CsvExport2Job,
        "name": "CSV_EXPORT2",
        "parser": csv_export_parser,
        "description": DESCRIPTION,
        "examples": EXAMPLES,
        "category": export
    }