adding kdoc preprocessing guide

KDOC_PREPROCESSING.md

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+# KDoc Preprocessing
+
+You might have spotted some notations like `{@include [Something]}` in the KDocs of DataFrame's source code.
+These are special notations for the [KDoc preprocessor](https://github.com/Jolanrensen/docProcessorGradlePlugin) 
+that we use to generate parts of the KDoc documentation. 
+
+Kotlin libraries like DataFrame use KDoc to document their code and especially their public API. This allows users
+to understand how to use the library and what to expect from it. However, writing KDoc can be a tedious task, especially
+when you have to repeat the same information in multiple places. The KDoc preprocessor allows us to write the information
+only once and then include it in multiple places.
+
+This document explains how to use the KDoc preprocessor in the DataFrame project.
+
+## How the Processing Works
+
+Unlike Java, Kotlin library authors
+[don't have the ability to share a jar file with documentation](https://github.com/Kotlin/dokka/issues/2787). They have
+to share their documentation along with their `sources.jar` file which users can attach in their IDE to see the docs.
+DataFrame thus uses the preprocessor in Gradle to copy and modify the source code, processing the KDoc notations,
+and publishing the modified files as the `sources.jar` file.
+
+This can be seen in action in the `core:processKDocsMain` and `core:changeJarTask` Gradle tasks in the 
+[core/build.gradle.kts file](core/build.gradle.kts). When you run any `publish` task in the `core` module, the
+`processKDocsMain` task is executed first, which processes the KDocs in the source files and writes them to the 
+`generated-sources` folder. The `changeJarTask` task then makes sure that any `Jar` task in the `core` module uses the
+`generated-sources` folder as the source directory instead of the normal `src` folder.
+
+`core:processKDocsMain` can also be run separately if you just want to see the result of the KDoc processing.
+
+To make sure the generated sources can be seen and reviewed on GitHub, since [PR #731](https://github.com/Kotlin/dataframe/pull/731),
+there's a [GitHub action](.github/workflows/generated-sources.yml) that runs the `core:processKDocsMain` task and
+comments the results on the PR. After a PR is merged, [another action](.github/workflows/generated-sources-master.yml)
+runs on the master branch and commits the generated sources automatically.
+This way, the generated sources are always up-to-date with the latest changes in the code.
+This means you don't have to run and commit the generated sources yourself, though it's
+still okay if you do.
+
+## Previewing the Processed KDocs in IntelliJ IDEA
+
+## The Notation
+
+## KDoc Preprocessor Conventions in DataFrame
+
+## KDoc -> WriterSide