Update docs with how users can add unknown keywords themselves

[tim-vd-aardweg]

What is the need for this task.
The documentation should be updated with how users can add support for custom keywords that are not yet supported, for example:

class RandomSection(INIBasedModel):
    class Comments(INIBasedModel.Comments):
        randomkeyword: Optional[str] = Field("This is a random keyword", alias="randomKeyword")

    _header: Literal["Geometry"] = "RandomSection"
    comments: Comments = Comments()

    unknownkeywordinunknownsection: str = Field("default", alias="UnknownKeywordInUnknownSection")

class ExtendedGeometry(Geometry):
    newkeyword: int = Field(0, "newKeyword")
    unknownkeyword: int = Field(0, alias="unknownKeyword")
    unknownkeywordwithcomment: str = Field("", alias="unknownKeywordWithComment")

class ExtendedFMModel(FMModel):
    geometry: ExtendedGeometry = Field(default_factory=ExtendedGeometry)
    randomsection: RandomSection = Field(default_factory=RandomSection)

input_mdu = (
        test_input_dir / "dflowfm_individual_files" / "with_unknown_keywords.mdu"
)

model = ExtendedFMModel(input_mdu)