Add certbot-pkcs12 to list of third-party plugins (#10488)

Closes #10348.

## Pull Request Checklist

- [ ] The Certbot team has recently expressed interest in reviewing a PR
for this. If not, this PR may be closed due our limited resources and
need to prioritize how we spend them.
- [ ] If the change being made is to a [distributed
component](https://certbot.eff.org/docs/contributing.html#code-components-and-layout),
add a description of your change to the `newsfragments` directory. This
should be a file called `<title>.<type>`, where `<title>` is either a
GitHub issue number or some other unique name starting with `+`, and
`<type>` is either `changed`, `fixed`, or `added`.
* For example, if you fixed a bug for issue number 42, create a file
called `42.fixed` and put a description of your change in that file.
- [ ] Add or update any documentation as needed to support the changes
in this PR.
- [ ] Include your name in `AUTHORS.md` if you like.

certbot/docs/using.rst

@@ -332,6 +332,7 @@ dns-mijn-host_          Y    N    DNS Authentication using mijn.host DNS
 nginx-unit_             Y    Y    Automates obtaining and installing a certificate with Nginx Unit
 dns-cdmon_              Y    N    DNS Authentication using cdmon's API
 dns-synergy-wholesale_  Y    N    DNS Authentication using Synergy Wholesale DNS
+pkcs12_                 N    Y    Install certificates as PKCS#12 archives
 ======================= ==== ==== =================================================================
 
 .. _haproxy: https://github.com/greenhost/certbot-haproxy
@@ -365,6 +366,7 @@ dns-synergy-wholesale_  Y    N    DNS Authentication using Synergy Wholesale DNS
 .. _nginx-unit: https://github.com/kea/certbot-nginx-unit
 .. _dns-cdmon: https://github.com/rascazzione/certbot-dns-cdmon
 .. _dns-synergy-wholesale: https://github.com/ALameLlama/certbot-dns-synergy-wholesale
+.. _pkcs12: https://github.com/nasa-gcn/certbot-pkcs12
 
 If you're interested, you can also :ref:`write your own plugin <dev-plugin>`.