Show creating extension before using

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/15/citext.html
Description:

On https://www.postgresql.org/docs/current/citext.html in "F.10.2. How to
Use It", it says "here's a simple example of usage" and begins by creating a
table with a `CITEXT` column.

For seasoned PostgreSQL users, it may be obvious that before running such an
example, one must `CREATE EXTENSION IF NOT EXISTS citext;`, but if a newer
user is told "use citext for this column" and given a link to this page,
nothing tells them how to enable it.

I suggest adding this instruction somewhere on this page, either in its own
section like "How to Enable It", or at the start of the "simple example", or
even at the end of the sentence near the top: "It can be installed by
non-superusers who have CREATE privilege on the current database by running
`CREATE EXTENSION IF NOT EXISTS citext;`."