docs: update docs for monorepo - #10317

The basics

• I validated my changes

The details

Resolves

Towards #7772

Proposed Changes

Updates docs to reflect monorepo changes, including:

• Reorganizes contributing docs so that "plugins" are part of the "core" section, and "plugins" section just becomes one page about examples
• Updates the plugins docs to remove references to blockly-samples
• Edits down add_a_plugin.mdx which was too verbose
• Fixes references to blockly-samples elsewhere in the guides, where appropriate
• Changes the debugging.mdx page to remove npm link workflow and add the new workflow
• Adds info about new versioning system to add_a_plugin.mdx and plugin_overview.mdx
• Updates README to remove info about codelabs/plugins being in blockly-samples

Reason for Changes

• plugins are now in the blockly repository
• new versioning system

Test Coverage

n/a

Documentation

n/a

Additional Information

After this PR, there is just one unresolved docs update in #7772, which is to add information about configuring dependabot to update blockly/plugins together.