Added information about SSL-certificates

In the meeting there were talks about Sectigo vs Let's Encrypt, I believe that the following part contains enough information:

Why should I choose either

If your service needs the full certificate chain to prove that your website belongs to the University of Helsinki, then Sectigo is preferable. If any SSL certificate works, Let's Encrypt is the easier solution. The difference is that Sectigo verifies organization ownership yearly. This means that any certificate signed by them means that the certificate was generated by the organization the certificate says it belongs to. On the other hand, Let's Encrypt does not and will not verify organization ownership, so there is no way to be sure that the website belongs to who it claims. For more information about certificate chains, 1 and 2.

Closes https://version.helsinki.fi/tike-kontit/tiksupankki/-/issues/93