Rename Available Commands to Main Commands (#4975)

## Changes
Rename command group `Available Commands` to `Main Commands`.

With the rename, if only one command group survives filtering, drop it
so Cobra can default to `Available Commands:` again.

## Why
If `Management Commands` and/or `Permission Commands` groups are
present, it is a bit confusing from `--help` that they are also
available (because customers might stop parsing the list after the
`Available Commands` heading group ends)

before:
```
% databricks permissions --help
Usage:
  databricks permissions [flags]
  databricks permissions [command]

Available Commands
  get                   Get object permissions.
  set                   Set object permissions.
  update                Update object permissions.

Permission Commands
  get-permission-levels Get object permission levels.
```

after:
```
% ./cli permissions --help
Usage:
  databricks permissions [flags]
  databricks permissions [command]

Main Commands
  get                   Get object permissions.
  set                   Set object permissions.
  update                Update object permissions.

Permission Commands
  get-permission-levels Get object permission levels.

Flags:
  -h, --help   help for permissions
```

## Tests
manual, see Why section above ^

---------

Co-authored-by: simon <4305831+simonfaltum@users.noreply.github.com>

Author: janniklasrose

acceptance/cmd/unknown-subcommand/output.txt

@@ -9,7 +9,7 @@ Usage:
   databricks secrets [flags]
   databricks secrets [command]
 
-Available Commands
+Available Commands:
   create-scope  Create a new secret scope.
   delete-acl    Delete an ACL.
   delete-scope  Delete a secret scope.

acceptance/cmd/workspace/secrets/output.txt

@@ -15,7 +15,7 @@ Usage:
   databricks secrets [flags]
   databricks secrets [command]
 
-Available Commands
+Available Commands:
   create-scope  Create a new secret scope.
   delete-acl    Delete an ACL.
   delete-scope  Delete a secret scope.

acceptance/pipelines/databricks-cli-help/output.txt

@@ -18,7 +18,7 @@ Usage:
   databricks pipelines [flags]
   databricks pipelines [command]
 
-Available Commands
+Main Commands
   deploy                Deploy pipelines
   destroy               Destroy a pipelines project
   dry-run               Validate correctness of the pipeline's graph

cmd/cmd.go

@@ -34,9 +34,18 @@ const (
 )
 
 // configureGroups adds groups to the command, only if a group
-// has at least one available command.
+// has at least one available command. When only one group survives
+// filtering, the grouping is dropped so Cobra's default "Available
+// Commands" heading is used — matching commands that don't define
+// groups at all.
 func configureGroups(cmd *cobra.Command, groups []cobra.Group) {
 	filteredGroups := cmdgroup.FilterGroups(groups, cmd.Commands())
+	if len(filteredGroups) <= 1 {
+		for _, sub := range cmd.Commands() {
+			sub.GroupID = ""
+		}
+		return
+	}
 	for i := range filteredGroups {
 		cmd.AddGroup(&filteredGroups[i])
 	}
@@ -85,7 +94,7 @@ func New(ctx context.Context) *cobra.Command {
 		groups := []cobra.Group{
 			{
 				ID:    mainGroup,
-				Title: "Available Commands",
+				Title: "Main Commands",
 			},
 			{
 				ID:    pipelines.ManagementGroupID,