docs(docs-infra): add instructions on using Let's Encrypt for preview server certificates (angular#47250)

Add a new doc with instructions on how to use TLS certificates issues by
[Let's Encrypt][1] in the PR preview server.

[1]: https://letsencrypt.org/

PR Close angular#47250

Author: gkalpak

aio/aio-builds-setup/docs/_TOC.md

@@ -30,4 +30,5 @@
 
 ## Miscellaneous
 - [Debug docker container](misc--debug-docker-container.md)
+- [Use Let's Encrypt certificates](misc--use-lets-encrypt-certs.md)
 - [Integrate with CI](misc--integrate-with-ci.md)

aio/aio-builds-setup/docs/misc--use-lets-encrypt-certs.md

@@ -0,0 +1,80 @@
+# Miscellaneous - Use Let's Encrypt TLS certificates
+
+
+[Let's Encrypt](https://letsencrypt.org/) is a free, automated, and open certificate authority (CA),
+provided by the [Internet Security Research Group (ISRG)](https://www.abetterinternet.org/). It can
+be used for issuing the certificates needed by the preview server. See the "Create TLS certificates"
+section in [this doc](vm-setup--create-host-dirs-and-files.md) for more details.
+
+Let's Encrypt supports issuing
+[wildcard certificates](https://letsencrypt.org/docs/faq/#does-let-s-encrypt-issue-wildcard-certificates),
+but they have to be renewed every 3 months using a
+[DNS-01 challenge](https://letsencrypt.org/docs/challenge-types/#dns-01-challenge).
+
+Below are instructions for the creation of certificates using Let's Encrypt.
+
+**Note 1:**
+  It is assumed that the commands are executed on the VM that hosts the preview server. You can run
+  the commands on any machine and adjust accordingly the steps for transfering the generated
+  certificate and private key to the preview server VM.
+
+**Note 2:**
+  In order to complete the DNS-01 challenge and generate the certificate, one needs to have access
+  to update the DNS records associated with the `ngbuilds.io` domain and subdomains.
+
+
+## Prerequisites
+- Install [certbot](https://certbot.eff.org/):
+  - Old method: `sudo apt-get install certbot`
+  - New method: Follow the instructions [here](https://certbot.eff.org/instructions).
+
+
+## Create new certificate
+- Run the following command and follow the on-screen instructions:
+  ```sh
+  sudo certbot certonly \
+    -d "ngbuilds.io,*.ngbuilds.io" \
+    -m "devops@angular.io" \
+    --agree-tos \
+    --manual \
+    --manual-public-ip-logging-ok \
+    --preferred-challenges="dns"
+  ```
+
+  **Hint:**
+    You can use `dig -t txt _acme-challenge.ngbuilds.io` (on Linux/macOS) or something like
+    [DNSChecker.org](https://dnschecker.org/all-dns-records-of-domain.php?query=_acme-challenge.ngbuilds.io&rtype=TXT) to verify that the DNS updates have been successfully deployed and propagated.
+
+- [Optional] Remove the DNS TXT records added in the previous step.
+  (They are no longer needed, unless you want to create more certificates now.)
+
+
+## Deploy new certificate
+- Copy files to the host machine's `/etc/ssl/localcerts` directory, replacing `YYYY-MM` in the file
+  names with the current year and month (for example, `2022-08`):
+  ```sh
+  # Copy certificate.
+  sudo cp /etc/letsencrypt/live/ngbuilds.io/fullchain.pem /etc/ssl/localcerts/ngbuilds.io.crt.YYYY-MM
+  sudo cp /etc/letsencrypt/live/ngbuilds.io/fullchain.pem /etc/ssl/localcerts/ngbuilds.io.crt
+
+  # Copy private key.
+  sudo cp /etc/letsencrypt/live/ngbuilds.io/privkey.pem /etc/ssl/localcerts/ngbuilds.io.key.YYYY-MM
+  sudo cp /etc/letsencrypt/live/ngbuilds.io/privkey.pem /etc/ssl/localcerts/ngbuilds.io.key
+
+  # Update permissions.
+  sudo find /etc/ssl/localcerts -type f -exec chmod 400 {} \;
+  ```
+
+  **Note:**
+    The `.YYYY-MM`-suffixed copies are not needed/used. They are just kept for reference and backup
+    purposes.
+
+- Reload `nginx`:
+  ```sh
+  # Connect to the Docker container and reload reload `nginx`.
+  sudo docker exec -it aio service nginx reload
+  ```
+
+- [Optional] Verify that the certificate has been successfully deployed by visiting
+  https://ngbuilds.io/ in a browser and inspecting the TLS certificate details. See, for example,
+  [how to view certificate details in Chrome](https://www.howtogeek.com/292076/how-do-you-view-ssl-certificate-details-in-google-chrome/).

aio/aio-builds-setup/docs/vm-setup--create-host-dirs-and-files.md

@@ -32,6 +32,10 @@ specify the domain name see [here](vm-setup--create-docker-image.md).
 If no directory is attached, nginx will use an internal self-signed certificate. This is convenient
 during development, but is not suitable for production.
 
+Certificates from any certificate authority can be used. As an example, see
+[here](misc--use-lets-encrypt-certs) for instructions on using certificates from
+[Let's Encrypt](https://letsencrypt.org/).
+
 **Note:**
 Since nginx needs to be able to serve requests for both the main domain as well as any subdomain
 (e.g. `ngbuilds.io/` and `foo-bar.ngbuilds.io/`), the provided certificate needs to be a