Acknowledge supporters and removed PayPal links

Since 2013, there had been the option to support PediTools via PayPal donations. In February 2018, I made the decision to include advertising support on PediTools to offset development and maintenance costs.

As of 2022, the advertising revenue has been sufficient to support PediTools, so I've decided to remove all links to PayPal from the site.

However, I did want to thank and acknowledge the 32 individuals who supported PediTools (including 3 who provided monthly donations!), and have added them to an updated About PediTools page.

Many thanks to all of you!