Add MySQL image instructions.

content/anvil/connecting_to_mysql.md

static/images/security_groups_1.png

static/images/security_groups_2.png

static/images/security_groups_3.png

static/images/security_groups_4.png

static/images/security_groups_5.png

static/images/security_groups_6.png

static/images/security_groups_7.png

static/images/security_groups_8.png

static/images/security_groups_9.png

static/images/sql_server_1.png

static/images/sql_server_2.png

static/markdown/anvil-images.md