doc, charset: add a chapter about how to add a new charset into tidb (#31848)
doc, charset: add a chapter about how to add a new charset into tidb (#31848)

ref pingcap/tidb#31690
c4pt0r/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,ngaut/tidb,pingcap/tidb,c4pt0r/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,pingcap/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,kyle-wang/tidb,ngaut/tidb,ngaut/tidb,c4pt0r/tidb,kyle-wang/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,c4pt0r/tidb,pingcap/tidb