doc: add internal links

Author: cusma

docs/units.md

@@ -29,7 +29,8 @@ If the D-ASA has a *principal*, its initial *unit value* **MUST** be equal to th
 The D-ASA *unit* *value* **MAY** change over time.
 
 > The D-ASA unit’s value may change according to different conditions, such as an
-> amortizing principal repayment schedule (see Amortizing Schedule section).
+> amortizing principal repayment schedule (see [Amortizing Schedule](./principal-repayment.md#amortizing-schedule)
+> section).
 
 The D-ASA *unit value* **MAY** change globally or locally (per account).