Have our own reference pages instead of links.

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.
   In the functions-and-operators directory, there are many pages that simply uses a link to dev.mysql.com/doc/refman/... instead of explaining the function/operator in our own documentation. This also causes a dependency on an external resource.

2. Describe your suggestion or addition.
   Create content describing the various functions and operators in TiDB, instead of simply linking to the mysql reference manual.

3. Provide some reference materials (such as documents and websites) if you could.

$  git grep 'dev.mysql.com.*refman' | cut -d: -f1 | uniq -c | sort -n -r
  75 functions-and-operators/operators.md
  62 functions-and-operators/date-and-time-functions.md
  59 functions-and-operators/string-functions.md
  38 functions-and-operators/numeric-functions-and-operators.md
  35 functions-and-operators/json-functions.md
  19 functions-and-operators/miscellaneous-functions.md
  13 functions-and-operators/information-functions.md
  12 functions-and-operators/window-functions.md
  12 functions-and-operators/encryption-and-compression-functions.md
  12 functions-and-operators/aggregate-group-by-functions.md
  11 functions-and-operators/expressions-pushed-down.md
   8 functions-and-operators/bit-functions-and-operators.md

Start by running the listed git grep command to inspect external MySQL reference links in the functions-and-operators directory. Read the affected pages, including operators.md, date-and-time-functions.md, string-functions.md, and the other listed function pages, to determine the scope. Done means the relevant TiDB function and operator content is documented locally instead of relying on those links.