Refactor render system (#32492)

There were too many patches to the Render system, it's really difficult
to make further improvements.

This PR clears the legacy problems and fix TODOs.

1. Rename `RenderContext.Type` to `RenderContext.MarkupType` to clarify
its usage.
2. Use `ContentMode` to replace `meta["mode"]` and `IsWiki`, to clarify
the rendering behaviors.
3. Use "wiki" mode instead of "mode=gfm + wiki=true"
4. Merge `renderByType` and `renderByFile`
5. Add more comments

----

The problem of "mode=document": in many cases it is not set, so many
non-comment places use comment's hard line break incorrectly

routers/web/user/profile.go

@@ -258,7 +258,6 @@ func prepareUserProfileTabData(ctx *context.Context, showPrivate bool, profileDb
 					Base:       profileDbRepo.Link(),
 					BranchPath: path.Join("branch", util.PathEscapeSegments(profileDbRepo.DefaultBranch)),
 				},
-				Metas: map[string]string{"mode": "document"},
 			}, bytes); err != nil {
 				log.Error("failed to RenderString: %v", err)
 			} else {