(docs) add RFC file to introduce Notebook entity data model

docs/rfc/active/4237-datadoc-entity/datadoc-entity-rfc.md

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+- Start Date: (fill me in with today's date, 2022-02-22)
+- RFC PR: https://github.com/linkedin/datahub/pull/4237 
+- Discussion Issue: (GitHub issue this was discussed in before the RFC, if any)
+- Implementation PR(s): (leave this empty)
+
+# Extend data model to model Notebook entity 
+
+## Background
+[Querybook](https://www.querybook.org/) is Pinterest’s open-source big data IDE via a notebook interface. 
+We(Included Health) leverage it as our main querying tool. It has a feature, DataDoc, which organizes rich text, 
+queries, and charts into a notebook to easily document analyses. People could work collaboratively with others in a 
+DataDoc and get real-time updates. We believe it would be valuable to ingest the DataDoc metadata to Datahub and make
+it easily searchable and discoverable by others.
+
+## Summary
+This RFC proposes the data model used to model DataDoc entity. It does not talk about any architecture, API or other 
+implementation details. This RFC only includes minimum data model which could meet our initial goal. If the community 
+decides to adopt this new entity, further effort is needed.
+
+## Detailed design
+
+### DataDoc Model
+![DataDoc High Level Model](DataDoc-high-level-model.png) 
+
+As shown in the above diagram, DataDoc is a document which contains a list of DataDoc cells. It organizes rich text,
+queries, and charts into a notebook to easily document analyses. We could see that the DataDoc model is very similar as 
+Notebook. DataDoc would be viewed as a subset of Notebook. Therefore we are going to model Notebook rather than DataDoc. 
+We will include "subTypes" aspect to differentiate Notebook and DataDoc 
+
+### Notebook Data Model
+This section talks about the mininum data model of Notebook which could meet our needs.
+- notebookKey (keyAspect)
+    - notebookTool: The name of the DataDoc tool such as QueryBook, Notebook, and etc
+    - notebookId: Unique id for the DataDoc
+- notebookInfo
+    - title(Searchable): The title of this DataDoc
+    - description(Searchable): Detailed description about the DataDoc
+    - lastModified: Captures information about who created/last modified/deleted this DataDoc and when
+- notebookContent
+    - content: The content of a DataDoc which is composed by a list of DataDocCell
+- editableDataDocProperties
+- ownership
+- status
+- globalTags
+- institutionalMemory
+- browsePaths
+- domains 
+- subTypes
+- dataPlatformInstance
+- glossaryTerms
+
+### Notebook Cells
+Notebook cell is the unit that compose a Notebook. There are three types of cells: Text Cell, Query Cell, Chart Cell. Each 
+type of cell has its own metadata. Since the cell only lives within a Notebook, we model cells as one aspect of Notebook
+rather than another entity. Here are the metadata of each type of cell:
+- TextCell
+    - cellTitle: Title of the cell
+    - cellId: Unique id for the cell.
+    - lastModified: Captures information about who created/last modified/deleted this Notebook cell and when
+    - text: The actual text in a TextCell in a Notebook
+- QueryCell
+    - cellTitle: Title of the cell
+    - cellId: Unique id for the cell.
+    - lastModified: Captures information about who created/last modified/deleted this Notebook cell and when
+    - rawQuery: Raw query to explain some specific logic in a Notebook
+    - lastExecuted: Captures information about who last executed this query cell and when
+- ChartCell
+    - cellTitle: Title of the cell
+    - cellId: Unique id for the cell.
+    - lastModified: Captures information about who created/last modified/deleted this Notebook cell and when
+
+## Future Work
+Querybook provides an embeddable feature. We could embed a query tab which utilize the embedded feature in Datahub 
+which provide a search-and-explore experience to user.