Use Pelican content dir relative links

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add screenshots to guide

 *  Empty list of service integrations
 *  A list with single service integration

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Configure Markdown via an extension

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Use fatal_error if home page URL is invalid

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Guide: explain home_url field is optional

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Guide: describe app runner and web worker

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
guide: add App Runner Process section

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