website: Update details on homepage and sponsor page

[jemgillam]

Homepage needs updating:

• Grafast is now open source
• Worker has its own documentation website
• PostGraphile has its own website too!

Sponsors page needs updating:

• Grafast is no longer sponsors only

[benjie]

Don't forget https://build.graphile.org and https://star.graphile.org also

[jemgillam]

Support page needs updating - calls are cheaper

[jemgillam]

@benjie should we add RITA to our graphile.org homepage?

[benjie]

Probably!