Add `io.Closer` guidelines (#29387)

Co-authored-by: Yarden Shoham <[e-mail]>
(cherry picked from commit ad0a34b492c3d41952ff4648c8bfb7b54c376151)

docs/content/contributing/guidelines-backend.en-us.md

···
 Since there are some packages which use the same package name, it is possible that you find packages like `modules/user`, `models/user`, and `services/user`. When these packages are imported in one Go file, it's difficult to know which package we are using and if it's a variable name or an import name. So, we always recommend to use import aliases. To differ from package variables which are commonly in camelCase, just use **snake_case** for import aliases.
 i.e. `import user_service "code.gitea.io/gitea/services/user"`
 
+### Implementing `io.Closer`
+
+If a type implements `io.Closer`, calling `Close` multiple times must not fail or `panic` but return an error or `nil`.
+
 ### Important Gotchas
 
 - Never write `x.Update(exemplar)` without an explicit `WHERE` clause:

modules/git/blame.go

···
 
 // Close BlameReader - don't run NextPart after invoking that
 func (r *BlameReader) Close() error {
+	if r.bufferedReader == nil {
+		return nil
+	}
+
 	err := <-r.done
 	r.bufferedReader = nil
 	_ = r.reader.Close()

modules/git/repo_base_gogit.go

···
 }
 
 // Close this repository, in particular close the underlying gogitStorage if this is not nil
-func (repo *Repository) Close() (err error) {
+func (repo *Repository) Close() error {
 	if repo == nil || repo.gogitStorage == nil {
-		return
+		return nil
 	}
 	if err := repo.gogitStorage.Close(); err != nil {
 		gitealog.Error("Error closing storage: %v", err)
 	}
+	repo.gogitStorage = nil
 	repo.LastCommitCache = nil
 	repo.tagCache = nil
-	return
+	return nil
 }
 
 // GoGitRepo gets the go-git repo representation

modules/git/repo_base_nogogit.go

···
 	}
 }
 
-func (repo *Repository) Close() (err error) {
+func (repo *Repository) Close() error {
 	if repo == nil {
 		return nil
 	}
···
 	}
 	repo.LastCommitCache = nil
 	repo.tagCache = nil
-	return err
+	return nil
 }

modules/indexer/internal/bleve/indexer.go

···
 }
 
 func (i *Indexer) Close() {
-	if i == nil {
+	if i == nil || i.Indexer == nil {
 		return
 	}
 

modules/indexer/internal/meilisearch/indexer.go

···
 	if i == nil {
 		return
 	}
-	if i.Client == nil {
-		return
-	}
 	i.Client = nil
 }

modules/util/filebuffer/file_backed_buffer.go

···
 	if b.file != nil {
 		err := b.file.Close()
 		os.Remove(b.file.Name())
+		b.file = nil
 		return err
 	}
 	return nil