chore: use GitHub actions to sanitize themes & plugins list

[stevenjoezhang]

I have improved a script I wrote earlier that can automatically check whether the links for themes and plugins have been updated or become invalid, and it periodically opens a pull request to update them.

See #1458 (comment)

[github-actions]

Theme thumbnails validation completed.

[stevenjoezhang]

We previously discussed the issue of theme preview websites becoming unavailable (in #1941 (comment)), and currently, dozens of theme preview websites are either down or redirected, with some even being maliciously registered due to expired domains.
The main problem now is that it requires a significant amount of manual effort to determine whether these invalid links are due to slow access speeds leading to timeouts, or if they are no longer in use. I was thinking that perhaps we could use the now-discontinued hexojs.github.io domain to deploy preview sites for third-party themes. (See https://github.com/orgs/hexojs/discussions/5409)

[stevenjoezhang]

Another issue is that there are more than 60 repositories of themes or plugins are archived. I suggest that for repositories that are archived or have not been updated for a certain period (for example, 3 years), we need to provide a notice on the hexo.io website that the theme may no longer be available, or just remove them.

[uiolee]

use the now-discontinued hexojs.github.io domain to deploy preview sites for third-party themes.

If really necessary, it is better to use the repository page hexojs.github.io/<repo>/. At the same time, code security need to be considered.

theme preview websites becoming unavailable

To avoid this, I recommend that request the authors to use github action to build previews site in their own github page. We can add guide to https://hexo.io/docs/themes#Publishing