docs: Add a social media share icon

[KentarouTakeda]

Hi all,

Thank you for reviewing the Japanese translation. I tried to post about its publication on social media, but:

X:

Facebook: NOTE: In the preview, no image is displayed, but upon actual sharing, the Netlify logo image at the bottom of the page appears, which may not be suitable

Since the image was not displayed, the post ended up looking very plain 😅

Therefore, I propose setting a specific page on the Hexo Official Website as the Open Graph Image and Twitter Summary Card image.

After implementing this pull request, posts on social media will display as follows:

NOTE: These images were generated with OGP Image Simulator and may be slightly different from the actual image.

X:

Facebook:

Previously, only the index page displayed the thumbnail of the landscape theme

Although that behavior was unintentional, to prevent it from being replaced by this pull request, we should explicitly designate the landscape image on the index page as the image for social media.

Thank you for your attention.

Check List

Please read and check followings before submitting a PR.

• [x] Others (Update, fix, translation, etc...)
  • Languages:
  • [x] en English
  • [x] ru Russian
  • [x] zh-cn simplified Chinese
  • [x] zh-tw traditional Chinese
  • [x] ja Japanese

[github-actions[bot]]

Theme name validation passed. Theme thumbnails validation completed.