Update installation instructions in readme

This changes the installation instructions in README.md to
recommend "pip install ." instead of "python setup.py install". The
former is compatible with Python 3.12 which doesn't have setuptools
installed by default (so setup.py, which imports it, can be
indirectly but not directly used). This also matches the
corresponding change made in the installation unit test.

While doing so, I've also clarified the instructions, and added the
implied "cd" command as well as the "git fetch --tags" command in
the position where a later section was recently updated to mention
it should have been run.

Using "pip install ." creates the opportunity to pass "-e" to make
an editable install, which users who clone the repository to work
on changes should do, because the effect of an editable install is
only partially simulated by pytest, and so that manual testing of
changes actually uses the changes intended for testing.

This increases the length and detail of the instructions, so I've
added h4 subsections to clarify the separations between them and
make it easier for readers to find the part they're looking for. In
doing so, I've reordered these subsections accordingly. Because
greater detail can create the impression that all important steps
are mentioned, I've made the general good advice to use a virtual
environment explicit. For brevity, I have not added venv commands.

Author: EliahKagan

README.md

@@ -49,30 +49,49 @@ The installer takes care of installing them for you.
 
 ### INSTALL
 
-If you have downloaded the source code:
+GitPython and its required package dependencies can be installed in any of the following ways, all of which should typically be done in a [virtual environment](https://docs.python.org/3/tutorial/venv.html).
 
-```bash
-python setup.py install
-```
+#### From PyPI
 
-or if you want to obtain a copy from the Pypi repository:
+To obtain and install a copy [from PyPI](https://pypi.org/project/GitPython/), run:
 
 ```bash
 pip install GitPython
 ```
 
-Both commands will install the required package dependencies.
+(A distribution package can also be downloaded for manual installation at [the PyPI page](https://pypi.org/project/GitPython/).)
+
+#### From downloaded source code
+
+If you have downloaded the source code, run this from inside the unpacked `GitPython` directory:
 
-A distribution package can be obtained for manual installation at: <http://pypi.python.org/pypi/GitPython>.
+```bash
+pip install .
+```
 
-If you like to clone from source, you can do it like so:
+#### By cloning the source code repository
+
+To clone the [the GitHub repository](https://github.com/gitpython-developers/GitPython) from source to work on the code, you can do it like so:
 
 ```bash
 git clone https://github.com/gitpython-developers/GitPython
-git submodule update --init --recursive
+cd GitPython
+git fetch --tags
 ./init-tests-after-clone.sh
 ```
 
+If you are cloning [your own fork](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/about-forks), then replace the above `git clone` command with one that gives the URL of your fork. Or use this [`gh`](https://cli.github.com/) command (assuming you have `gh` and your fork is called `GitPython`):
+
+```bash
+gh repo clone GitPython
+```
+
+Having cloned the repo, create and activate your [virtual environment](https://docs.python.org/3/tutorial/venv.html). Then make an [editable install](https://pip.pypa.io/en/stable/topics/local-project-installs/#editable-installs):
+
+```bash
+pip install -e .
+```
+
 ### Limitations
 
 #### Leakage of System Resources