Update `WhatsNext`page for core plugin terminology changes

[pavlenex]

The https://docs.btcpayserver.org/WhatsNext/ page needs to be updated to reflect the removal of the distinction between Apps and Plugins.

As explained in issue #1559, BTCPay Server no longer separates Apps and Plugins. What used to be called apps are now core system plugins. From the user’s perspective, features such as Point of Sale and Crowdfunding are now presented as plugins in the sidebar.
T
he goal of this task is straightforward: remove references to apps and make sure the page consistently refers to Crowdfunding, Point of Sale, and Pay Button as plugins, within the plugins section.

The refactored page should include:

• A recommendation to try the Payment Requests feature for cases where the user wants to send an invoice or payment link to a client or a friend.
• A recommendation to explore our e-commerce integrations section, which already exists.
• A plugins section that introduces Point of Sale, Pay Button, and Crowdfunding as the default starting points, while also mentioning that users can explore additional plugins to extend functionality.
• A note that merchants can submit their business to the public, voluntary merchant directory: https://directory.btcpayserver.org
• A Join the Community section.

If you’d like to work on this issue, leave a comment and I’ll assign it to you.

[BLOCKNERD-lab]

I would like to solve this issue

[pavlenex]

Go ahead @BLOCKNERD-lab!

[DisGrid]

If it's still available
I'd like to solve this issue

[pavlenex]

Go for it @DisGrid when you have a PR I'll assign it you. For rest of PR's you commented on, let's go one at a time and tackle them once we close this one, to allow other chance