Update PF6 upgrade guide with additional details.

[edonehoo]

The PF6 upgrade guide could use a few updates as described in #4454, #4446, #4442, and some Slack conversations. Address the linked issues and the following details within one PR:

• Link to https://www.patternfly.org/tokens/develop-with-tokens/#react-tokens
• Add mention of the EmptyState changes, which are currently only outlined in https://www.patternfly.org/get-started/release-highlights#component-updates-0
• Explicitly list removed or deprecated components (chip, tile, ?)