doc: git-add: clarify intro & add an example

doc: git-add: clarify intro & add an example

- Add a basic example of how "git add" is normally used
- It's not technically true that you *must* use the `add` command to
  add changes before running `git commit`, because `git commit -a`
  exists. Instead say that you *can* use the `add` command.
- Mention early on that "index" is another word for "staging area",
  since Git very rarely uses the word "index" in its output
  (`git status`) uses the term "staged", and many Git users are
  unfamiliar with the term "index"
- Remove "It typically adds" (it's not clear what "typically" means),
  and instead mention that `git add -p` can be used to add
  partial contents
- Currently the introduction is somewhat repetitive ("to prepare the
  content staged for the next commit" ... "this snapshot that is taken
  as the contents of the next commit."), replace with a single sentence
  ("The "index" [...] is where Git stores the contents of the next
  commit.")

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