[IMP] general integrations: 19.2 mail plugins update

[fw-bot]

documentation task card: https://www.odoo.com/odoo/my-tasks/5978429

update mail plugin docs for 19.2:

• mail plugins article: removed pricing section and its lead gen/iap subsection
• outlook plugin article:
  • removed module installation section
  • replaced XML installation → marketplace installation
• gmail plugin article: removed module installation section

outstanding questions to address:

• Leads: are they still part of the featureset?
  • Excalidraw + task chatter say "opportunities, tickets, leads"
  • Microsoft marketplace copy still mentions leads
• Module installed by default:
  • Excalidraw says "the module should be installed by default when you install either of those: CRM, Helpdesk, Project, Sales"
  • Do we need to account for edge cases?
• On-prem section?:  on-prem instructions are very different, have those changed to marketplace? Or does it stay the same?

This saas-19.2 PR can be FWP up to master.

Forward-Port-Of: #16575

[robodoo]

[fw-bot]

@huisit @Felicious this PR targets master and is the last of the forward-port chain.

To merge the full chain, use

@robodoo r+

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port