docs: add ADR for fixed USD pricing

docs/decisions/0028-fixed-usd-pricing.rst

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+28. Fixed USD Pricing
+========================
+
+Status
+--------
+Accepted (Jul 2024)
+
+Context
+---------
+Course Discovery is equipped with the capability to ingest and update courses from external sources (through the csv_loader).
+These external courses may be priced in various currencies, necessitating conversion to a standardized currency (USD) before
+their introduction into discovery. Given the volatility of exchange rates, the product prices coming into discovery may fluctuate with
+each data ingestion.
+
+The regular fluctuation in prices poses challenges for customers, especially those purchasing courses in bulk (B2B). These fluctuations can lead to
+situations where a customer plans to make a purchase within budget, only to find that prices have changed unfavorably when
+they proceed with the transaction. This inconsistency may disrupt budgetary planning and affect decision-making processes for
+customers. This is particularly problematic for B2B customers, who may have predefined contracts and can not exceed their budget constraints. 
+
+Decision
+----------
+A new field, fixed_price_usd, will be introduced to the course run model within Course Discovery. This field will be accessible
+through the standard course and search APIs. Consumers will have the ability to retrieve and utilize this field from Discovery APIs,
+enabling them to price their offerings based on this stable fixed price in USD. It is assumed that the fixed price will remain
+relatively stable over time, minimizing the impact of currency fluctuations for B2B customers.
+
+The existing pricing fields and their associated logic will remain unchanged within Course Discovery. It will be the responsibility
+of consumers, such as enterprise, to appropriately manage any potential implications of using the fixed price field within their own
+systems and processes.
+
+Consequences
+--------------
+- The current Pricing flow in Discovery will remain as is.
+- There will be no impact on Seats, Entitlements, and their associated logic.
+- The current flow of propagation of pricing information to Ecommerce will remain as is.
+- Analytics/Data teams will need to ensure that they track Enterprise Customers' purchases against the fixed price appropriately.
+
+Alternatives Considered
+-------------------------
+
+- It would have been preferable to add this field at the Seat level somehow, and not clutter the CourseRun model unnecessarily.
+  However, due to some historical reasons, Executive Education courses (for which we need this functionality), only have unpaid
+  seats. It would be confusing to add pricing information to an unpaid seat.
+
+- The field could have been integrated at the course level, aligning with the current discovery flows where the course price (CourseEntitlement)
+  is applied to all course runs. However, this approach does not accommodate pricing at the course run level, which has been a significant
+  issue for discovery users in the past. Although there are some workarounds, such as manually adjusting prices in the database, these solutions
+  are both cumbersome and inefficient.