[mlir][docs] Update/Add documentation for MLIRs Pattern Rewrite infrastructure

This infrastructure has evolved a lot over the course of MLIRs lifetime, and has never truly been documented outside of rationale or proposals. This revision aims to document the infrastructure and user facing API, with the rationale specific portions moved to the Rationale folder and updated.

Differential Revision: https://reviews.llvm.org/D85260

Author: River707

mlir/docs/DialectConversion.md

@@ -151,12 +151,12 @@ target.markOpRecursivelyLegal<MyOp>([](MyOp op) { ... });
 ## Rewrite Pattern Specification
 
 After the conversion target has been defined, a set of legalization patterns
-must be provided to transform illegal operations into legal ones. The structure
-of the patterns supplied here is the same as those described in the
-[quickstart rewrites guide](Tutorials/QuickstartRewrites.md#adding-patterns).
-The patterns provided do not need to generate operations that are directly legal
-on the target. The framework will automatically build a graph of conversions to
-convert non-legal operations into a set of legal ones.
+must be provided to transform illegal operations into legal ones. The patterns
+supplied here have the same structure and restrictions as those described in the
+main [Pattern](PatternRewriter.md) documentation. The patterns provided do not
+need to generate operations that are directly legal on the target. The framework
+will automatically build a graph of conversions to convert non-legal operations
+into a set of legal ones.
 
 As an example, say you define a target that supports one operation: `foo.add`.
 When providing the following patterns: [`bar.add` -> `baz.add`, `baz.add` ->