[Disk Manager] add dm codestyle doc (#1894)

* add dm codestyle doc

* eng

doc/disk_manager/codestyle.md

@@ -0,0 +1,25 @@
+## Main Part
+
+The code style is based on https://go.dev/doc/effective_go and https://github.com/golang/go/wiki/CodeReviewComments, but there are some exceptions.
+
+## Exceptions that will not be refactored
+
+- Fixed line length of 80 characters
+- Mandatory empty newlines at the beginning of each function if the signature is multiline
+- Nested errors are discouraged
+- By default, nil value receivers are not supported for methods, and this is not explicitly checked
+- Underscores are allowed in package names
+- Comments for exported objects are not mandatory
+- Comments do not need to start with the name of the object being commented on
+- Indent after `}` (exception: defer is pressed against the block above without indentation, for example: https://github.com/ydb-platform/nbs/blob/9c6622e69132261561cf582d4c6d8ad4fdccd67f/cloud/disk_manager/pkg/admin/disks.go#L99)
+- Indent before `)` in the case of multiline calls (example: https://github.com/ydb-platform/nbs/blob/9c6622e69132261561cf582d4c6d8ad4fdccd67f/cloud/disk_manager/pkg/admin/images.go#L123)
+- Comments are formatted as valid sentences: starting with a capital letter and ending with a period
+- Use ID in all identifiers (not Id) (except for protobufs)
+
+## Exceptions that can be refactored throughout the code and removed later
+
+- The constructor is not placed directly below the structure declaration
+- Constructors are not mandatory for classes (not talking about data container classes like Point, but rather logical ones like Storage)
+- Named function result arguments are allowed wherever desired, not only where they add readability to the API
+- Writer functions to channels are not logically separated from Readers in some multithreaded code
+- Mixing receiver arguments in struct methods is allowed