fix(docs): add link to monetization section of Nextcloud documentation

[edward-ly]

To be added in conjunction with these PRs:

• feat: add "Request quote" button for enterprise support on eligible apps #1449
• feat: add donation attribute to appinfo/info.xml, donation buttons to app details page #1452
• feat(developer): add section for monetizing apps documentation#12108

[github-actions]

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