[docs] When can you parse without the `CustomParseFormat` plugin?

Nobody has claimed this yet.

**Describe the bug**
[String + Format](https://day.js.org/docs/en/parse/string-format) (https://github.com/dayjs/dayjs-website/blob/master/docs/parse/string-format.md) does not specify when the `CustomParseFormat` plugin can be omitted. The second line "This requires the `CustomParseFormat` plugin to work" makes it seem like you should always use the plugin.

**Expected behavior**
List all cases where you can parse without the `CustomParseFormat` plugin. ChatGPT reports these cases:
```
YYYY-MM-DD
YYYY-MM-DDTHH:mm:ssZ
YYYY-MM-DDTHH:mm:ss.SSSZ
YYYY-MM-DDTHH:mm
YYYY-MM-DDTHH
YYYY-MM
YYYY
MM-DD
HH:mm:ss
HH:mm
```

**Information**
- Day.js version: 1.11.13
- OS: N/A
- Browser: N/A
- Time zone: N/A

Start with docs/parse/string-format.md and the linked String + Format documentation, then inspect how the CustomParseFormat plugin affects the listed formats. Verify which cases work without the plugin before updating the explanation to identify them clearly. Done means the documentation no longer implies that the plugin is always required.

Written by the indexing model from the issue text.