CASSANDRA-19777: Update doc page for how-to commit with more intro

[arvindKandpal-ksolves]

Why is this PR being submitted?

This PR resolves CASSANDRA-19777.

New committers often need a quick overview of the expected Git workflow before diving into the specific patch/branch-based contribution steps. This PR adds a brief introduction to the how_to_commit documentation to outline these prerequisites, as requested in the Jira ticket, without turning the page into a full Git tutorial.

What changes were made?

Added a new == Introduction for New Committers section to how_to_commit.adoc which covers:

• The preference for using the Git CLI over the GitHub Web UI.
• The requirement to use local feature branches and never push a branch directly upstream.
• The squash-rebase-merge workflow.
• Essential git commands (remote add, rebase -i, commit --amend, push --atomic -n).
• The GitHub CLI commands for locally testing and merging a PR.

[Copilot]

Pull request overview

Updates Cassandra’s committer documentation (how_to_commit.adoc) to add a short “Introduction for New Committers” section that summarizes expected Git workflow prerequisites before the existing branch/patch-based contribution instructions.

Changes:

• Adds an introductory section describing preferred tooling (CLI), use of local feature branches, and a high-level workflow summary.
• Adds a short list of “basic git commands” and an example sequence intended to show how to locally test and merge a PR.

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[arvindKandpal-ksolves]

Hi @bschoening , I have applied Copilot suggestions , Please take a look.