Add Discord icon to navbar

[mario4tier]

Look into a way to add a discord icon/link prior to the github icon on the top navbar:

This can probably be implemented with the "Using Slots" feature described here:
See https://theme-hope.vuejs.press/guide/advanced/replace.html

If not possible, then consider using our new HTML post-processing feature ( #35 )

[mario4tier]

This is how theme-hope did it for a telegram icon in the navbar:

https://github.com/search?q=repo%3Avuepress-theme-hope%2Fvuepress-theme-hope%20SocialLink&type=code