[docs] cleanup README and add coverage badge (aptos-labs#2936)

Author: geekflyer

README.md

@@ -4,12 +4,9 @@
 
 ---
 
-[![Aptos Rust Crate Documentation (main)](https://img.shields.io/badge/docs-main-59f)](https://aptos-labs.github.io/aptos-core/)
 [![License](https://img.shields.io/badge/license-Apache-green.svg)](LICENSE)
 [![Lint+Test](https://github.com/aptos-labs/aptos-core/actions/workflows/lint-test.yaml/badge.svg)](https://github.com/aptos-labs/aptos-core/actions/workflows/lint-test.yaml)
-[![grcov](https://img.shields.io/badge/Coverage-grcov-green)](https://ci-artifacts.aptoslabs.com/coverage/unit-coverage/latest/index.html)
-[![test history](https://img.shields.io/badge/Test-History-green)](https://ci-artifacts.aptoslabs.com/testhistory/aptos/aptos/auto/ci-test.yml/index.html)
-[![Automated Issues](https://img.shields.io/github/issues-search?color=orange&label=Automated%20Issues&query=repo%3Aaptos%2Faptos%20is%3Aopen%20author%3Aapp%2Fgithub-actions)](https://github.com/aptos-labs/aptos-core/issues/created_by/app/github-actions)
+[![codecov](https://codecov.io/gh/aptos-labs/aptos-core/branch/main/graph/badge.svg?token=X01RKXSGDE)](https://codecov.io/gh/aptos-labs/aptos-core)
 [![Discord chat](https://img.shields.io/discord/945856774056083548?style=flat-square)](https://discord.gg/aptoslabs)
 
 Aptos-core strives towards being the safest and most scalable layer one blockchain solution. Today, this powers the Aptos Devnet, tomorrow Mainnet in order to create universal and fair access to decentralized assets for billions of people.
@@ -19,11 +16,10 @@ Aptos-core strives towards being the safest and most scalable layer one blockcha
 * [Aptos Labs](https://aptoslabs.com/)
 * [Aptos Developer Network](https://aptos.dev)
 * [Getting Started](https://aptos.dev/guides/getting-started)
-* [Life of a Transaction](https://aptos.dev/guides/basics-life-of-txn)
 * Join us on the [Aptos Discord](https://discord.gg/aptoslabs).
 
 ## Contributing
 
 You can learn more about contributing to the Aptos project by reading our [Contribution Guide](https://github.com/aptos-labs/aptos-core/blob/main/CONTRIBUTING.md) and by viewing our [Code of Conduct](https://github.com/aptos-labs/aptos-core/blob/main/CODE_OF_CONDUCT.md).
 
-Aptos Core is licensed as [Apache 2.0](https://github.com/aptos-labs/aptos-core/blob/main/LICENSE).
+Aptos Core is licensed under [Apache 2.0](https://github.com/aptos-labs/aptos-core/blob/main/LICENSE).