Add Order Factory page and split Contingent Orders

[AlexCatarino]

Follow-up to #2710 for QuantConnect/Lean#9828.

• Adds Order Management › Order Factory. Order Tickets and Transaction Manager move to 02 and 03, and their URLs don't change.
• Splits Contingent Orders into a landing page with Bracket, OCO, OTO, OUO, and Custom pages. Shared sections live in Resources/order-types/contingent-orders/.
• Moves the fee estimate and buying power check into shared includes used by the Order Factory page, Transaction Fees Key Concepts (new 05 Estimate Fees), and Buying Power › Get Initial Margin Requirements. The old initial margin check accepted every sell order, because the margin is negative for sells, and rejected strategies that close an existing position.
• Adds an example to Transaction Fees and one to Buying Power, and fixes the =ggt; typo and the protective call/put mismatch in Buying Power Example 1.

Merge after QuantConnect/Lean#9828. The examples that use the order factory aren't marked testable yet, and the new pages' og:image files don't exist.

Test plan

• docs-style linter passes on all new and changed pages, and every changed .php section renders without warnings.
• The fee estimate and buying power checks match LEAN's own decisions in local backtests on the current release, with hand-built requests in place of the order factory: equity orders that reverse a position, and Option spreads that open or close positions.
• All contingent order images load from the CDN.