Add Introduction to apps.md and restructure

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Restructure Service Integrations dropdown

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add doc Core Development

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Tutorial: Add info about custom actions

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Tutorial: Add links to API references

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>