Readme updates 2026-Feb-21 part 2 (#721)

* Move optional steps for testing VMs out of README.md file
This may help avoid new users being confused that they must follow
those instructions.

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

* Add 2025-Jan-30 versions of source code for install.sh

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

* Update the README table-of-contents to match content

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

---------

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

Author: jafingerhut

README.md

@@ -6,11 +6,7 @@
 * [Introduction](#introduction)
 * [Presentation](#presentation)
 * [P4 Documentation](#p4-documentation)
-* [P4sim](#p4sim--a-simple-p4-behavioral-simulator)
 * [Obtaining required software](#obtaining-required-software)
-     * [To build the virtual machine](#to-build-a-virtual-machine-with-the-p4-development-tools)
-     * [Accessing the VM](#download-a-virtual-machine-with-the-p4-development-tools-already-installed)
-     * [To install P4 development tools on an existing system](#to-install-p4-development-tools-on-an-existing-system)
 * [How to Contribute](#how-to-contribute)
 * [Older tutorials](#older-tutorials)