Add Frappe Suite to the JMAP clients list

[s-aga-r]

Summary

• Add Frappe Suite to the Clients section of the software page. It is an open-source (AGPL-3.0) productivity suite whose Mail and Calendar apps are JMAP clients: users read and send mail and manage calendars against the mail server over JMAP.
• Its JMAP layer is built on jmaplib, which is proposed for the Libraries section in Add jmaplib to the JMAP libraries list #68.
• The entry is placed alphabetically, after Flectar Mail and before Group-Office.

Test plan

• npx prettier --check pages/software.liquid passes.
• The page front matter parses as YAML and the new item sits in the Clients section.