@W-21352368 Technical documentation for web applications

[lesya7]

What does this PR do?

Adds a webApplicationsSourceAdapter section to HANDBOOK.md for the WebApplication metadata type. Per review, the README Web Applications section and docs/WEBAPPLICATION.md were removed; user-facing docs live in plugin-app-dev.

What issues does this PR fix or reference?

#, @W-21352368@

Functionality Before

No WebApplication documentation in HANDBOOK.

Functionality After

HANDBOOK.md includes a webApplicationsSourceAdapter section that documents the bundle structure, required files, and example layout.

[jshackell-sfdc]

Hi @lesya7,

The WebApplication metadata type is just one of many types that SDR handles, so we keep the docs at a high-level very general.

Here are our requests:

• Remove the Web Applications section in the README.
• Also remove the new docs/WEBAPPLICATION.md file.
• Move relevant info in this PR to the HANDBOOK.MD file in this SDR repo. Take a look at the entry for digitalExperienceAdapter for an example of the type of info that belongs in the handbook.
• Move any remaining information in this PR that you want to keep to the plugin-app-dev repo. This is "your" repo so you can update docs as you like.

Hope that makes sense!

FYI @soridalac , @iowillhoit

[jshackell-sfdc]

Small editing tweak. @iowillhoit , do you need to review this update to the SDR handbook too?

[iowillhoit]

@lesya7 Can you please do a quick update on the PR description to reflect the new state (for future reference)

[lesya7]

@iowillhoit Done.