docs: Polish SkyWalking doc and a tiny thing about Zipkin

[wu-sheng]

What this PR does / why we need it:

Neither Zipkin nor SkyWalking is a part of OpenTracing ecosystem, especially when we speak from Nginx and LUA perspectives. Also, OpenTracing is being abandoned, their APIs are not widely used, and merged into OpenTelemetry, which also doesn't relate to the Nginx ecosystem. So, let's focus on the tech itself.

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

[wu-sheng]

Why CI get canceled?

[juzhiyuan]

Why CI get canceled?

See https://github.com/apache/apisix/tree/master/.github/actions

[wu-sheng]

Why CI get canceled?

See https://github.com/apache/apisix/tree/master/.github/actions

SkyWalking has this, but never cancels CI for the latest commit. What do I miss?

[juzhiyuan]

Why CI get canceled?

See https://github.com/apache/apisix/tree/master/.github/actions

SkyWalking has this, but never cancels CI for the latest commit. What do I miss?

got it. 😳 You mean the last commit's checks are cancelled unexpectedly, need @Yiyiyimu to confirm.

[juzhiyuan]

AFAIK, we have paths_ignore in most checks for docs changes, do they have a relationship? 🤔

Those checks won't run.

[Yiyiyimu]

AFAIK, we have paths_ignore in most checks for docs changes, do they have a relationship? 🤔

Since this PR also changes README, so all CI would be running. Still checking the reason.

---

I guess a part of the reason is that this PR is not coming from a forked repo, like most PRs. The rule of cancel-workflow-action is kind of sophisticated, and just got noticed that It got deprecated since Github provides an official feature about it so I guess I would not fix it (since for this PR doc change does not need all CI passed), but try to apply the new one.