webcache deprecation, fixes #1495

[rfay]

The Problem/Issue/Bug:

We're no longer supporting the experimental webcache feature (macOS only)

How this PR Solves The Problem:

• Make it less obtrusive in the config.yaml (don't show it unless it's turned on)
• Add minor docs about it in the comment section of config.yaml
• Turn off webcache-specific testing except for the one TestWebcache test.
• Make clear in docs (readthedocs) that it's deprecated and what it might be used for.

Manual Testing Instructions:

• ddev config with webcache_enabled: false should make it disappear from the main top section of config.yaml
• ddev config should create an explanatory section at bottom of config.yaml
• Read docs (https://github.com/rfay/ddev/blob/c0746749649f004e22a5c7f3afbbab4d49db9f37/docs/users/performance.md#using-webcache_enabled-to-cache-the-project-directory-deprecated-macos-only)

Related Issue Link(s):

OP #1495
@kaystrobach's #1528 PR

Release/Deployment notes: