Explain the docs' CD, relating to the milestone the doc is set to vs online docs

What does this MR do?

Docs dev guidelines on /help: explain that the docs are continuously deployed every hour, while /help docs are shipped within a given milestone.

This is constantly asked, therefore, should be explained in the docs :)

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards