[close #328] cdc: simplify readme

[pingyu]

Signed-off-by: Ping Yu yuping@pingcap.com

What problem does this PR solve?

Issue Number: close #328

Problem Description: Simplify README

What is changed and how does it work?

• Remove user manual and replace with links to tikv.org.
• Add image of architecture.
• Fix badge of codecov.

Rendered here.

Code changes

• No code

Check List for Tests

This PR has been tested by at least one of the following methods:

• No code

Side effects

• No side effects

Related changes

• No related changes

To reviewers

Please follow these principles to check this pull requests:

• Concentration. One pull request should only do one thing. No matter how small it is, the change does exactly one thing and gets it right. Don't mix other changes into it.
• Tests. A pull request should be test covered, whether the tests are unit tests, integration tests, or end-to-end tests. Tests should be sufficient, correct and don't slow down the CI pipeline largely.
• Functionality. The pull request should implement what the author intends to do and fit well in the existing code base, resolve a real problem for TiDB/TiKV users. To get the author's intention and the pull request design, you could follow the discussions in the corresponding GitHub issue or internal.tidb.io topic.
• Style. Code in the pull request should follow common programming style. (For Go, we have style checkers in CI). However, sometimes the existing code is inconsistent with the style guide, you should maintain consistency with the existing code or file a new issue to fix the existing code style first.
• Documentation. If a pull request changes how users build, test, interact with, or release code, you must check whether it also updates the related documentation such as READMEs and any generated reference docs. Similarly, if a pull request deletes or deprecates code, you must check whether or not the corresponding documentation should also be deleted.
• Performance. If you find the pull request may affect performance, you could ask the author to provide a benchmark result.

(The above text mainly refers to TiDB Development Guide. It's also highly recommended to read about Writing code review comments)

[pingyu]

@zeminzhou @haojinming PTAL, thanks~

[codecov]

Codecov Report

Merging #327 (fa2b180) into main (0966ba8) will decrease coverage by 0.1675%.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##               main       #327        +/-   ##
================================================
- Coverage   61.0409%   60.8734%   -0.1676%     
================================================
  Files           240        240                
  Lines         20270      20334        +64     
================================================
+ Hits          12373      12378         +5     
- Misses         6777       6832        +55     
- Partials       1120       1124         +4     

Flag	Coverage Δ		*Carryforward flag
br	60.4647% <ø> (ø)		Carriedforward from 492b05d
cdc	61.0616% <ø> (-0.2460%)	⬇️

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
cdc/cdc/sorter/unified/heap_sorter.go	84.1836% <0.0000%> (-3.0613%)	⬇️
cdc/cdc/kv/client.go	83.8616% <0.0000%> (-2.1614%)	⬇️
cdc/pkg/orchestrator/etcd_worker.go	77.3109% <0.0000%> (-1.2606%)	⬇️
cdc/cdc/server.go	35.1851% <0.0000%> (-0.8338%)	⬇️
cdc/cdc/http_router.go	81.4814% <0.0000%> (-0.5186%)	⬇️
cdc/cdc/sorter/unified/file_backend.go	33.3333% <0.0000%> (-0.4387%)	⬇️
cdc/cdc/sorter/unified/merger.go	68.2847% <0.0000%> (-0.3237%)	⬇️
cdc/cmd/cdc/main.go	100.0000% <0.0000%> (ø)
cdc/cdc/capture/capture.go	0.0000% <0.0000%> (ø)
cdc/cdc/kv/region_worker.go	79.5833% <0.0000%> (ø)
... and 2 more

[pingyu]

/run-integration-test

[pingyu]

/run-integration-test

[pingyu]

/run-integration-test

[zeminzhou]

/run-integration-test

[haojinming]

LGTM

[zeminzhou]

LGTM~