doc: rewrite and restructure doc/P4sim/README.md

[Mingyumaz]

Summary

This PR rewrites and restructures doc/P4sim/README.md to improve clarity, fix errors, and make it suitable for the p4lang official repository.

Changes

• Rewrote the introduction and key features list for clarity
• Added a full step-by-step local installation guide (Ubuntu 24.04, ns-3.39 or earlier) with bash code blocks
• Added explanation for why ns-3.39 or earlier is required (C++17 vs C++20 constraint with bmv2)
• Added P4SIM_DIR environment variable setup instructions
• Added mailto: links for maintainer and contributor emails
• Restored missing references in Publications

[github-actions]

Welcome to the project, @Mingyumaz!

It looks like this is your first contribution. We noticed the DCO (Developer Certificate of Origin) check might fail if your commits aren't signed.

To fix this, please ensure every commit has a Signed-off-by: Name <email> line. You can do this automatically by using the -s flag:
git commit -s -m "Your message"

For existing commits, you can fix them with:
git commit --amend --signoff or git rebase -i HEAD~N --signoff (where N is the number of commits)
The Developer Community DCO guide also provides helpful tips on fixing DCO inconveniences. Setting a commit hook in the git repository will automate adding the DCO signoff.
See https://github.com/p4lang/governance/wiki/P4-DCO-Guidelines for information.

[Dscano]

Could you please add links to the specs?

[jafingerhut]

Linking to this page for all P4.org specifications should be good, unless you want to link to a particular version of one of the spec documents: https://p4.org/specifications/

[Dscano]

I was thinking of linking to the latest version of each document, but your proposal is actually better.

[jafingerhut]

LGTM

[jafingerhut]

I have tried out these instructions, they worked for me on an x86_64 Ubuntu 24.04 system as written without any problems, so I am approving and will merge these in. It would be nice to have links to the spec documents, but I am fine with those links being added with a later PR.