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

[yetanothertw]

The section looks so much better and easier to read with the added tables and extra context. Thank you for fixing it up! :)

I can see what you mean about it being tricky to combine
all the tables into one. But I think the last table that reflects availability for each action and phase combo is very useful and complements the standalone phase-focused tables.

[yetanothertw]

A more general comment about action capitalization. The linked references all use sentence case, like Set priority instead of Set Priority. Should we be using the same in the tables here?

There's another discrepancy with hyphenation in Read-Only and [Read only](https://www.elastic.co/docs/reference/elasticsearch/index-lifecycle-actions/ilm-readonly) -- the latter should probably be Read-only.

[kilfoyle]

For sure! Thanks a lot for catching that.