Add sections on using @cost and @listSize to demand control docs

[tninesling]

Updates the demand control documentation to include details on @cost and @listSize for more accurate cost estimation.

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Note any exceptions here

Notes

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[shorgi]

lgtm!

[chandrikas]

We need to remove/change all references to this feature being in preview (The blurb at the top of the demand control page, the config parameter etc)

[chandrikas]

While reading these docs again in their entirety, I find them really long and hard to navigate. Can we turn these sections into links that the reader can directly navigate to?

Follow this workflow to configure and tune demand control for your router:

Measure the cost of your existing operations.
Improve the cost estimation model.
Adjust your preview_demand_control configuration and enforce cost limits.

I appreciate that I can navigate from the sidebar but links in this menu would be nice too.

[tninesling]

We need to remove/change all references to this feature being in preview (The blurb at the top of the demand control page, the config parameter etc)

Agree! I'll do that in a separate PR