docs: remove website leftovers

The website content moved to the github.com/containerd/containerd.io
repository.

Commit da1fba0 removed all website-
related content, but there were some stray files left behind.

This patch removes those files, and updates the `.editorconfig` file
to only match Markdown files.

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

docs/.dockerignore

@@ -1,13 +1,8 @@
-[*]
+[*.md]
 end_of_line = lf
 charset = utf-8
 max_line_length = 80
 trim_trailing_whitespace = true
 insert_final_newline = false
-
-[*.{html,js,sh,sass,md,mmark}]
 indent_style = space
 indent_size = 2
-
-[Makefile]
-indent_style = tab