docs: lint Markdown

[jbampton]

Lint Markdown for rule:

• MD022/blanks-around-headings

What this PR does / why we need it:

Standardizes the Markdown files

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[jbampton]

cc @spacewander

It seems the linters are not working now.

Error message was:

ibiqlik/action-yamllint@v3 is not allowed to be used in apache/apisix. Actions in this workflow must be: created by GitHub, verified in the GitHub Marketplace, within a repository owned by apache or match the following: apache/, gradle/wrapper-validation-action, gradle/wrapper-validation-action@, peter-evans/create-pull-request@, dawidd6/action-download-artifact@, scacap/action-surefire-report@*.

[spacewander]

@jbampton
The policy of Apache has changed recent day. Maybe we can run yamllint directly, just like what we have done with misspell?

apisix/.github/workflows/spellchecker.yml

Line 19 in b72e2c5

find apisix doc bin t -not -path "t/toolkit/*.lua" -type f | xargs ./misspell -error

[jbampton]

We need to wait until #3196 is merged in before we can test this PR.

After #3196 is merged I will rebase.