feat: use the value provided by markdown extensions as the default value for the table of contents title (#131)

[Xiaokang2022]

1. When option toc_title is specified as a value, the specified value is used;
2. If no value is specified, the default value provided by markdown extensions will be used;
3. When markdown extensions does not provide any value, "Table of Contents" is used as the default;

It's important to note point 3 above, according to https://squidfunk.github.io/mkdocs-material/setup/extensions/python-markdown/#+toc.title, when no value is specified, mkdocs-material actually automatically calculates the default value based on the language of the page, rather than using "Table of Contents" as the default value directly (unless the language of the page is English). But for that, I don't have a way to get the value that mkdocs-material automatically calculates.

[timvink]

Nice work!