Fix up 'phase actions' section

[kilfoyle]

Based on feedback from Support, this PR:

• Clarifies the definitions of the hot and warm ILM phases, since users do indeed update indices during those phases.
• Adds a description for each phase action so people don't have to jump back and forth to the reference docs. I put this in tables for easier reading.
• Adds a table summarizing which actions are available in which phases.

Note: I wanted to have one large table showing all actions, their descriptions, and the phases in which they're available, but there are two problems:

• Markdown doesn't support column spanning, which makes the table unwieldly.
• The order the actions is important and it's not the same for each phase, so we need a separate table for each phase.

Closes: #1559

[github-actions]

🔍 Preview links for changed docs

• manage-data/lifecycle/index-lifecycle-management/index-lifecycle.md