docs: fix broken images, links, and typos across documentation

[sbhavani]

What does this PR do ?

Documentation fixes across multiple README files and user guides.

Changes

Fixed broken image paths (6 images)

• docs/user-guide/features/dist_optimizer.md
• docs/user-guide/features/custom_fsdp.md
• docs/user-guide/features/multi_token_prediction.md

Fixed markdown header hierarchy (17 headers)

• megatron/core/datasets/readme.md
• megatron/core/transformer/moe/README.md

Fixed broken TOC anchors + added cross-reference

• docs/llama_mistral.md - Fixed 4 broken anchor links, added Megatron Bridge reference

Fixed typos (11 typos)

• megatron/core/transformer/moe/README.md
• megatron/core/optimizer/cpu_offloading/README.md
• megatron/core/distributed/fsdp/src/README.md

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 @megatron-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 [email protected] or [email protected].

Merging your PR

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

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