docs: improve CONTRIBUTING docs

[iportman]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

Improve CONTRIBUTING docs

1. Define format of the embedded script
2. Add embedded script for 'git config'
3. Improve case for 'github'

Ⅱ. Does this pull request fix one issue?

NO, just improve docs.

Ⅲ. Why don't you add test cases (unit test/integration test)?

Unnecessary for docs.

Ⅳ. Describe how to verify it

See the change via any markdown reader

Ⅴ. Special notes for reviews

[funky-eyes]

LGTM Please register the author and PR information in two 1.5.0.md files under the Changes folder

[codecov-commenter]

Codecov Report

Merging #4163 (d189a4c) into develop (f901e08) will decrease coverage by 0.33%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop    #4163      +/-   ##
=============================================
- Coverage      49.75%   49.42%   -0.34%     
+ Complexity      3767     3759       -8     
=============================================
  Files            700      700              
  Lines          23626    23626              
  Branches        2917     2918       +1     
=============================================
- Hits           11755    11677      -78     
- Misses         10671    10756      +85     
+ Partials        1200     1193       -7     

Impacted Files	Coverage Δ
...java/io/seata/server/metrics/MeterIdConstants.java	0.00% <0.00%> (-100.00%)	⬇️
...ava/io/seata/server/metrics/MetricsSubscriber.java	18.84% <0.00%> (-53.63%)	⬇️
...er/src/main/java/io/seata/server/ServerRunner.java	50.00% <0.00%> (-7.70%)	⬇️
...erver/storage/file/session/FileSessionManager.java	51.82% <0.00%> (-5.11%)	⬇️
...o/seata/server/coordinator/DefaultCoordinator.java	47.84% <0.00%> (-1.08%)	⬇️
...torage/file/store/FileTransactionStoreManager.java	56.12% <0.00%> (-0.65%)	⬇️
...rage/redis/store/RedisTransactionStoreManager.java	64.16% <0.00%> (-0.42%)	⬇️
...tasource/sql/struct/cache/MysqlTableMetaCache.java	81.60% <0.00%> (+1.14%)	⬆️

[slievrly]

LGTM