docs: contributor guide retains template placeholders and develop branch

## Description

`CONTRIBUTING.md` still contains unexpanded `{project_name}` template values and instructs contributors to branch from and submit to `develop`.

On current `main` (`6752ff8da9b1ea85c8e91b27b0a64fef099f22f9`):

- the issue and pull-request links point to `cloudwego/{project_name}` and do not resolve to this repository
- the fork instruction names `cloudwego {project_name}`
- the example command uses `git checkout -b my-fix-branch develop`
- the final instruction targets `{project_name}:develop`

The repository default branch is `main`, and there is no `develop` branch in the upstream branch list. These placeholders and branch instructions have been unchanged since the initial commit.

## Expected behavior

The contributor guide should link to `cloudwego/eino-ext` and tell external contributors to branch from and target `main`.

## Suggested fix

Replace the repository placeholders with `eino-ext` and replace both `develop` references with `main`.

Open CONTRIBUTING.md on commit 6752ff8da9b1ea85c8e91b27b0a64fef099f22f9 and review the repository links, fork instruction, branch command, and final target. Replace the {project_name} references with eino-ext and update develop to main; done means the guide points to cloudwego/eino-ext and consistently targets main.

Written by the indexing model from the issue text.