system-databases: Refactor/Improve clarity (#3178)

[ti-srebot]

cherry-pick #3178 to release-4.0

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What is changed, added or deleted? (Required)

Fixes #2642

There are actually quite a few issues with the current system tables documentation:

• It organizes them into categories that don't match where they are in MySQL (System Databases section of manual is confusing #2642)
• These are all called "system tables", but they have unique namespaces. It makes it confusing because it's quite possible that there could be a table of the same name in mysql.* and information_schema.* in future. Information_schema and metrics_schema would more correctly be called "system views", but I've not changed the navigation for now, as it was recently renamed from "System Databases". Information_schema also has meaning because it is an SQL standard. "system tables" is not.
• Some information_schema tables have their own docs page. Others do not. I went with converting everything to one page per table for consistency. This makes it similar to the SQL Statements section, which is easy to follow.
• Several of the example outputs have not been updated for recent TiDB versions.
• Several of the example outputs do not match the commands (i.e. command= DESC, output = show create table)
• In the main information schema page: https://docs.pingcap.com/tidb/v4.0/system-table-information-schema -- several of the example tables are repeated. See for example "SLOW_QUERY table" and "ANALYZE_STATUS table" -- but there are several others such as the tikv_* and tidb_* tables (!!)
• The heading indent level was inconsistent on the main i_s page.
• The "TiDB system tables" page makes it hard to follow that this is the MySQL schema. It should say it contains both stuff for MySQL and TiDB

I have also removed the "category" metadata of these files, since this is being deprecated.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

Fixes #2642

[yikeke]

LGTM