[Hacktoberfest Idea] Clean up the documentation

[Superskyyy]

There could be spots and nits in our documentation (especially the main repo) that are inaccurate, which should be cleaned up and fixed.

The task is to find them out and submit a patch to the relevant repo to fix them. A focus should be put on misleading guidance and statements that are no longer true in the current SkyWalking version according to your hands-on experience after trying the project.

A PR that purely focuses on grammar is acceptable, but please remember to aggregate them into one PR instead of a stream of many PRs; otherwise, they will be treated as SPAM and unqualified for Hacktoberfest credits.

Prerequisite:
The first step before contributing documentation changes is to try out our project.
Below are a few options, 1 is the easiest, 2 and 3 require some technical background.

1. use the demo site http://demo.skywalking.apache.org/general username:skywalking password:skywalking
2. deploy and play with the skywalking showcase in your local docker environment https://github.com/apache/skywalking-showcase
3. deploy and play with an actual skywalking server in your local machine by following https://skywalking.apache.org/docs/main/next/en/setup/backend/backend-setup/
   Step3 allows deep customization and integration with our agents/ other projects, where you will most likely find some places to enhance.

[Superskyyy]

This task can be accepted by multiple contributors, there's no need to assign or convert to issue. An hacktoberfest accepted label will be attached to your PR once passing the review.