Make distinct sections for advanced patterns and extending click.

[Rowlando13]

sections added:

• extending click
• click concepts

sections moved:

• custom groups (advanced patterns -> extending click)
• command aliases (advanced patterns -> extending click)
• callback evaluation order (advanced patterns -> click concepts)
• global context access (advanced patterns -> basic commands, groups and context)
• detecting source parameters (advanced patterns -> basic commands, groups and context)