Minimize README contents

[megnvidia]

What does this PR do ?

• quick start is in doc. Putting link in readme rather than duplicate content
• created install.md moved and formatted readme/install to that file. Added TOC link to new file.
• stole About content from in process quick start, left news in(it is in the sglang), moved our quick start link after the news
• added overview.md file, moved readme content (megatron-LM -->ecosystem libraries) to overview, merged a bit of my draft overview content from before. Overview needs some technical massage, I have some points wrong in my descriptions.
• adding releasenotes.md and manipulating the TOC for compatibility with doc team structure template (SWDOCS&title=Peer+Review+Checklist)
• removing training section because content duplicates better placed: https://docs.nvidia.com/megatron-core/developer-guide/latest/user-guide/data-preparation.html and https://docs.nvidia.com/megatron-core/developer-guide/latest/user-guide/training-examples.html
• removing Parallelism Strategies section because content duplicates better placed: https://docs.nvidia.com/megatron-core/developer-guide/latest/user-guide/parallelism-guide.html
• moved roadmaps to release notes
• changed title of community and support to Resources to align with swdocs structure template

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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Pre-checks

• I want this PR in a versioned release and have added the appropriate Milestone (e.g., Core 0.8)
• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
• I have run the autoformatter.sh on my PR

Code review

The following process is enforced via the CODEOWNERS file for changes into megatron/core. For changes outside of megatron/core, it is up to the PR author whether or not to tag the Final Reviewer team.

For MRs into `main` branch

Feel free to message or comment the @mcore-oncall to help accelerate your merge into main. The less complex your PR is, the faster it will be approved and merged!

(Step 1): Add PR label Expert Review

(Step 2): Collect the expert reviewers reviews

1. Attach the Expert Review label when your PR is ready for review.
2. GitHub auto-assigns expert reviewers based on your changes. They will get notified and pick up your PR soon.

⚠️ Only proceed to the next step once all reviewers have approved, merge-conflict are resolved and the CI is passing.
Final Review might get declined if these requirements are not fulfilled.

(Step 3): Final Review

1. Add Final Review label
2. GitHub auto-assigns final reviewers based on your changes. They will get notified and pick up your PR soon.

(Optional Step 4): Cherry-pick into release branch

If this PR also needs to be merged into core_r* release branches, after this PR has been merged, select Cherry-pick to open a new PR into the release branch.

For MRs into `dev` branch

The proposed review process for `dev` branch is under active discussion.

MRs are mergable after one approval by either eharper@nvidia.com or zijiey@nvidia.com.

Merging your PR

Any member of core-adlr and core-nemo will be able to merge your PR.

[copy-pr-bot]

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[Phlip79]

/ok to test f23b303

[Phlip79]

/ok to test 389f28c

[greptile-apps]

Automatic reviews are disabled for this repository.

[Phlip79]

/ok to test 6814b66