doc: add an explanation of Git's data model

Git very often uses the terms "object", "reference", or "index" in its
documentation.

However, it's hard to find a clear explanation of these terms and how
they relate to each other in the documentation. The closest candidates
currently are:

1. `gitglossary`. This makes a good effort, but it's an alphabetically
    ordered dictionary and a dictionary is not a good way to learn
    concepts. You have to jump around too much and it's not possible to
    present the concepts in the order that they should be explained.
2. `gitcore-tutorial`. This explains how to use the "core" Git commands.
   This is a nice document to have, but it's not necessary to learn how
   `update-index` works to understand Git's data model, and we should
   not be requiring users to learn how to use the "plumbing" commands
   if they want to learn what the term "index" or "object" means.
3. `gitrepository-layout`. This is a great resource, but it includes a
   lot of information about configuration and internal implementation
   details which are not related to the data model. It also does
   not explain how commits work.

The result of this is that Git users (even users who have been using
Git for 15+ years) struggle to read the documentation because they don't
know what the core terms mean, and it's not possible to add links
to help them learn more.

Add an explanation of Git's data model. Some choices I've made in
deciding what "core data model" means:

1. Omit pseudorefs like `FETCH_HEAD`, because it's not clear to me
   if those are intended to be user facing or if they're more like
   internal implementation details.
2. Don't talk about submodules other than by mentioning how they
   relate to trees. This is because Git has a lot of special features,
   and explaining how they all work exhaustively could quickly go
   down a rabbit hole which would make this document less useful for
   understanding Git's core behaviour.
3. Don't discuss the structure of a commit message
   (first line, trailers etc).
4. Don't mention configuration.
5. Don't mention the `.git` directory, to avoid getting too much into
   implementation details

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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