autosummary: Enable autosummary_generate by default

[tk0miya]

Is your feature request related to a problem? Please describe.
The autosummary extension is mainly used to generate an autosummary-table and stub pages referred by the table. But stub pages generation is disabled by default now. So users need to enable it manually for each project.

Note: As a side effect, this change causes an increase in build time sligtly for project that uses only autosummary-tables. But I consider they are very less.

Describe the solution you'd like
Enable autosummary_generate by default.

Describe alternatives you've considered
No.

Additional context
No