add ed.md (#944)

* add ed.md

inspired by this very nice introductory blog post:
https://sanctum.geek.nz/arabesque/actually-using-ed/

* ed.md: fix linting

pages/common/ed.md

@@ -0,0 +1,35 @@
+# ed
+
+> The original Unix text editor.
+
+- Start ed, editing an empty document (which can be saved as a new file in the current directory):
+
+`ed`
+
+- Start ed, editing an empty document, with `:` as a command prompt indicator:
+
+`ed -p :`
+
+- Start ed editing an existing file (this shows the byte count of the loaded file):
+
+`ed -p : {{path/to/file}}`
+
+- Toggle the printing of error explanations. (By default, explanations are not printed and only a `?` appears):
+
+`H`
+
+- Add text to the current document. Mark completion by entering a period by itself in a new line:
+
+`a<Enter>{{text_to_insert}}<Enter>.`
+
+- Print the entire document (`,` is a shortcut to the range `1,$` which covers the start to the end of the document):
+
+`,p`
+
+- Write the current document to a new file (the filename can be omitted if `ed` was called with an existing file):
+
+`w {{filename.txt}}`
+
+- Quit ed:
+
+`q`