docs(ivy): Add README to indexer module (#31260)

Describe the indexer module for Angular compiler developers. Include
scope of analysis provided by the module and the indexers it targets as
first-party.

PR Close #31260

packages/compiler-cli/src/ngtsc/indexer/README.md

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+# `indexer`
+
+The `indexer` module generates semantic analysis about components used in an
+Angular project. The module is consumed by a semantic analysis API on an Angular
+program, which can be invoked separately from the regular Angular compilation
+pipeline.
+
+The module is _not_ a fully-featured source code indexer. Rather, it is designed
+to produce semantic information about an Angular project that can then be used
+by language analysis tools to generate, for example, cross-references in Angular
+templates.
+
+The `indexer` module is developed primarily with the
+[Kythe](https://github.com/kythe/kythe) ecosystem in mind as an indexing
+service.
+
+### Scope of Analysis
+
+The scope of analysis performed by the module includes
+
+-   indexing template syntax identifiers in a component template
+-   generating information about directives used in a template
+-   generating metadata about component and template source files
+
+The module does not support indexing TypeScript source code.