Documentation change

[Logofile]

This change includes the following:

• Restructures the landing page (_index.md) to simplify the layout and bring the table of contents to the left side
• Removes the optional keyword from examples in the editions/features.md topic
• In /editions/overview.md:
  • Clarifies that 2023 is the latest released edition
  • Adds missing package syntax from proto examples
  • Adds a reserved keyword to the proto examples to show the syntax difference between proto2/proto3 and editions
  • Corrects syntax for enums in proto examples
• Corrects the example from the list_people command’s unit tests in getting-started/gotutorial.md
• Replaces an outdated link to OKRs with a link to codelabs in /overview.md
• Adds information about ruby_package to /programming-guides/dos-donts.md
• Corrects misspellings in several topics
• Reorganizes the content in the presence table in /programming-guides/field_presence.md
• Corrects instances of [[] that should have been [ throughout C++ API documentation
• Minor text changes in /cpp/arenas.md
• Removes some outdated references to has_foo() being proto2-only in /cpp/cpp-generated.md
• Adds information about ruby_package to /reference/ruby/ruby-generated.md
• Removes reference to cross-domain mixing of protobuf gencode and runtime from the external docs, since it applies only internally

PiperOrigin-RevId: 644492325 Change-Id: I651afbc2d6049818a1b579b4d0e779779cb06c57