Add Polkadot icon to the website

[0x4Graham]

Add the polkadot icon to the following list of icons on the website

Can we also rework the sizing of the logo's so they are consistent please?

[kattylucy]

• Add polkadot
• Fix width/height to fit images more consistently on page

https://github.com/centrifuge/website/assets/51223655/63602869-8232-48b3-9915-9b8b47ac9c98

[0x4Graham]

Looks great! LGTM 🛳️🛳️🛳️