How to feed my JSON dataset in Keras for character level text classification
As mentioned by @Frankstr you want a tutorial on "character-level classification", not handwritten digit recognition as the one you have linked.

Character-level classification is typically done with an RNN or a 1D CNN. See this implementation of Character-level Convolutional Networks for Text Classification for example.

In order to convert your json data to x_train and y_train which can be fed to Keras, you can read it into a dictionary, extract keys and values separately, and binarize your class labels:

import json
from sklearn.preprocessing import LabelBinarizer

with open('data.json', 'r') as f:
    train = json.load(f)

y_train = list(train.values())

lb = LabelBinarizer()
x_train = lb.fit_transform(list(train.keys()))


Make sure your json is properly formed. The example you gave is missing commas after each value. It should be:

{"candidate00001": "graceful ones.",
 "candidate00002":"One more,Marvelous said, sounding royally bored from his seat.",
 "candidate00003":"She tired,Joe said, though not unkindly."
}