docs: correct contribution repository details

[dajiaohuang]

What type of PR is this?

docs

Check the PR title.

• This PR title matches the format: <type>(optional scope): <description>
• The description is user-oriented and clear.
• This PR is itself the required documentation update.

(Optional) Translate the PR title into Chinese.

docs: 更正贡献指南中的仓库信息

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

The contribution guide retained {project_name} placeholders in its issue, pull-request, fork, and target instructions. It also told contributors to branch from and target develop, but this repository uses main as its default branch and has no upstream develop branch.

This change replaces the placeholders with cloudwego/eino-ext and updates the branch instructions to main.

Validation:

• confirmed the repository default branch is main
• confirmed no upstream develop branch exists
• repository-wide Markdown scan reports 0 remaining placeholder links in this branch
• git diff --check

(Optional) Which issue(s) this PR fixes:

Fixes #966

(optional) The PR that updates user documentation:

This PR.