### Description
This PR adds a Google Calendar integration feature as described in issue
#872. Users can embed their own calendar using an iframe configured via
`_config.yml`, and the calendar is toggleable through a responsive
button. This component is styled to visually blend into the theme and
supports responsive display across desktop and mobile.
### Changes
- Added `calendar.liquid` in `_includes`
- Modified `about.md` to include the toggleable calendar block
- Introduced `calendar` config section in `_config.yml`
- Appended calendar-related styles in `_sass/_base.scss`
### Configuration
To enable this feature, add the following to `_config.yml`:
```yaml
calendar:
enabled: true
calendar_id: your_calendar_id@group.calendar.google.com
timezone: UTC
style: "border:0; width:100%; height:600px;"
---------
Signed-off-by: George Araújo <george.gcac@gmail.com>
Co-authored-by: dodo <349507644@qq.com>
Co-authored-by: George <31376482+george-gca@users.noreply.github.com>
Co-authored-by: George Araújo <george.gcac@gmail.com>
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