Let's Encrypt certificate needs to be renewed

Background

The Let's Encrypt certificate will expire on Dec 26 07:39:46 2018 GMT and must to be renewed before that date.

The website deployed by this repo supports encrypted communication through HTTPS using a TLS digital certificate provided by Let's Encrypt.

These certificates are valid only for three months, and thus need to be periodically renewed. In order to do so, follow these steps.

Untick "Force domains with SSL certificates to use HTTPS" on the page settings.
Clone and run the certboot tool to generate a new certificate:
```
git clone https://github.com/certbot/certbot
cd certbot && ./letsencrypt-auto certonly -a manual -d docs.buildstream.build
```
When the ACME challenge token is displayed, do not press enter yet.
Open the ci settings, expand Variables and update the value of the ACME_CHALLENGE variable with the new token.
Manually run a pipeline for master to force the website to be redeployed.
Once the pipeline finishes and thus the website is redeployed, go back to the terminal and press enter now to continue. This will generate new fullchain.pem and privkey.pem files.
On the domain settings replace Certificate (PEM) with updated fullchain.pem and Key (PEM) with updated privkey.pem.
Tick back "Force domains with SSL certificates to use HTTPS" on the page settings.
Update the due date for this issue for three months later.

It is possible to access https://docs.buildstream.build with no warnings displayed concerning the certificate.

Edited Sep 27, 2018 by Tiago Gomes