Update documentation for distance-based localization

[oddvarlia]

**Add some more details for how distance-based localization works and an illustration **

Resolves: #14300

[codecov-commenter]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
86	1	85	0

View the top 1 failed test(s) by shortest run time

tests/ert/ui_tests/gui/test_docs_screenshots.py::test_that_all_png_files_under_the_docs_folder_has_been_considered_for_testing

Stack Traces | 0.052s run time

source_root = PosixPath('.../work/ert/ert')

    def test_that_all_png_files_under_the_docs_folder_has_been_considered_for_testing(
        source_root: Path,
    ):
        docs_folder = source_root / "docs"
        pngs_files_in_docs = set(docs_folder.rglob("*.png"))
    
        considered_pngs = {
            file
            for pattern in (
                *PNGS_NOT_APPLICABLE_FOR_GENERATION,
                *PNGS_TESTED_FOR_CHANGE,
                *TODOS,
            )
            for file in source_root.glob(pattern)
        }
    
        uncategorized_png_files = {
            str(file.relative_to(source_root))
            for file in pngs_files_in_docs - considered_pngs
        }
    
        newline = "\n  - "
>       assert not uncategorized_png_files, (
            "The following uncategorised png file(s) have been detected under the docs "
            f"folder:\n  - {newline.join(uncategorized_png_files)}\n"
            "If this is a screenshot of the gui consider adding a screenshot test for it\n"
            "and add the filepath to PNGS_TESTED_FOR_CHANGE list.\n"
            "Alternatively, add the filepath to either the TODOS or\n"
            "PNGS_NOT_APPLICABLE_FOR_GENERATION lists"
        )
E       AssertionError: The following uncategorised png file(s) have been detected under the docs folder:
E           - .../getting_started/howto/illustrating_influence_range.png
E         If this is a screenshot of the gui consider adding a screenshot test for it
E         and add the filepath to PNGS_TESTED_FOR_CHANGE list.
E         Alternatively, add the filepath to either the TODOS or
E         PNGS_NOT_APPLICABLE_FOR_GENERATION lists
E       assert not {'.../getting_started/howto/illustrating_influence_range.png'}

.../ui_tests/gui/test_docs_screenshots.py:524: AssertionError

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[xjules]

there is already a section How it works

[xjules]

In distance-based localization, lateral (Euclidean) distance is estimated between the observation coordinate and field parameter location. This distance neglects vertical distance and uses purely horizontal one.

The implemented method ...