Add the integration guide for SendGrid as a reference to support sending emails

[csharplus]

Description of changeset:

A working example app.py for email integration is provided based on SendGrid, as listed below:

# app.py

from flask import Flask
from flask_mail import Mail, Message

app = Flask(__name__)
app.config["SECRET_KEY"] = "top-secret!"
app.config["MAIL_SERVER"] = "smtp.sendgrid.net"
app.config["MAIL_PORT"] = 587
app.config["MAIL_USE_TLS"] = True
app.config["MAIL_USERNAME"] = "apikey"
app.config[
    "MAIL_PASSWORD"
] = "<Get it from SendGrid>"
app.config["MAIL_DEFAULT_SENDER"] = "<Test sender email>"


with app.app_context():
    mail = Mail(app)
    msg = Message("Twilio SendGrid Test Email", recipients=["<Test recipient email>"])
    msg.body = "This is a test email!"
    msg.html = "<p>This is a test email!</p>"
    mail.send(msg)

To run this example, please get the MAIL_PASSWORD (i.e., API_KEY) following the instructions given by SendGrid at here, then run the following commands in a terminal:

$ python3 -m venv venv
$ source venv/bin/activate
(venv) $ pip install flask flask-mail
(venv) $ python3 app.py

Test Plan:
CI

Reviewers:
@JJJ000 @mengting1010

[csharplus]

@JJJ000 @mengting1010 PTAL

[mengting1010]

LGTM