Improve documentation on creating custom metadata

[MalteEbner]

Description

There is currently no documentation on creating custom metadata in the coco format easily. Creating it by hand is not easy, as you need to take care of the image_ids. https://docs.lightly.ai/getting_started/platform.html#custom-metadata

Thus it would be good to show how to create a custom metadata file for more common formats, e.g. from a pandas dataframe.

Furthermore, the utility function save_custom_metadata() should be documented. https://github.com/lightly-ai/lightly/blob/fb2fac9f8c2f211fa7b162ece530653f6b19d5f1/lightly/utils/io.py#L234

Example code

The script is fully working

import numpy as np
import pandas as pd

from lightly.utils import save_custom_metadata

df = pd.DataFrame({
 "filename_A": [0, 3],
  "filename_B": [2, 3],
  "filename_C": [0, 7],
})
OUTPUT_FILE = "custom_metadata.json"

# create a list of pairs of (filename, metadata)
custom_metadata = []
for filename, data in df.items():
    metadata = {
     'lat': float(data[0]),
     'long': float(data[1]),
     'dist': float(np.sqrt(data[1] * data[1] + data[0] * data[0]))
    }
    custom_metadata.append((filename, metadata))

# save custom metadata in the correct json format
save_custom_metadata(OUTPUT_FILE, custom_metadata)