Update content for how to install, maintain, and deploy Plone 6

[stevepiercy]

Update content for how to install, maintain, and deploy Plone 6.

Include sub-tasks for Plone 6 Classic and Plone 6 Frontend.

Current installation methods

These need definitions and descriptions, including when you would use one or another.

• buildout
• pip
• Docker
• tox
• mxdev
• node

Stories

• demonstration
  • demo sites
  • local installation only
• development, testing, and deployment

Existing entry points and documentation

• https://plone.org/get-started Install points to https://plone.org/download now
• https://plone.org/download This is fine.
• https://5.docs.plone.org/manage/installing/installation.html - Needs to be migrated to Plone 6.
• https://training.plone.org/5/plone_training_config/instructions.html Removed.
• https://pypi.org/project/Plone/
  

[stevepiercy]

Superseded by

• Clarify install methods to either "Create a project" or "Contribute to a package" #1598
• Migrate manage/* from Plone 5.2 to Plone 6 Docs #1327
• Remove pip install Plone from PyPI and warehouse Products.CMFPlone#3892
• Help installers choose a frontend #1353