gist: add page (#1389)

* Include gist page

* Fix linting errors

* Quote "hello world" in the echo commnad

* Change wording to make explanation clearer

* Further simplify descriptions of gist command

* Simplify description of the update gist command

pages/common/gist.md

@@ -0,0 +1,23 @@
+# gist
+
+> Upload code to https://gist.github.com.
+
+- Create a gist from any number of text files:
+
+`gist {{file.txt}} {{file2.txt}}`
+
+- Create a private gist with a description:
+
+`gist -p -d {{"A meaningful description"}} {{file.txt}} `
+
+- Read contents from STDIN and create a gist from it:
+
+`{{echo "hello world"}} | gist`
+
+- List your public and private gists:
+
+`gist -l`
+
+- Use the id from the gist URL to modify or include a file:
+
+`gist -u {{GIST_ID}} {{file.txt}}`