message: add docs badge, fix build badge
diff: -[![Build Status](https://travis-ci.org/tantivy-search/tantivy.svg?branch=main)](https://travis-ci.org/tantivy-search/tantivy)
  +[![Docs](https://docs.rs/tantivy/badge.svg)](https://docs.rs/crate/tantivy/)
  +[![Build Status](https://github.com/tantivy-search/tantivy/actions/workflows/test.yml/badge.svg)](https://github.com/tantivy-search/tantivy/actions/workflows/test.yml)
  [![codecov](https://codecov.io/gh/tantivy-search/tantivy/branch/main/graph/badge.svg)](https://codecov.io/gh/tantivy-search/tantivy)
  [![Join the chat at https://gitter.im/tantivy-search/tantivy](https://badges.gitter.im/tantivy-search/tantivy.svg)](https://gitter.im/tantivy-search/tantivy?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
  [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
  -[![Build status](https://ci.appveyor.com/api/projects/status/r7nb13kj23u8m9pj/branch/main?svg=true)](https://ci.appveyor.com/project/fulmicoton/tantivy/branch/main)
  [![Crates.io](https://img.shields.io/crates/v/tantivy.svg)](https://crates.io/crates/tantivy)
  ![Tantivy](https://tantivy-search.github.io/logo/tantivy-logo.png)
  