Add discord link to header

[khaitruong922]

Closes #59

[jamesmaa]

Do you know if it's possible to extend the theme so we limit the amount of copy+paste we do? https://squidfunk.github.io/mkdocs-material/customization/#extending-the-theme

[khaitruong922]

Due to how the header HTML is structured, to insert the Discord icon, there is no other partials to override other than header.html.
https://github.com/squidfunk/mkdocs-material/blob/master/src/templates/partials/header.html

Override blocks only allow us to insert at the start or end of the header, therefore it is not possible to insert the Discord icon which is in the middle of header.
https://squidfunk.github.io/mkdocs-material/customization/#overriding-blocks

[jamesmaa]

Can you fix the broken link checker by adding it to lycheeignore?