Add codespell support and fix a good number of typos with its help (#3270)

More about codespell: https://github.com/codespell-project/codespell .

I personally introduced it to dozens if not hundreds of projects already and so far only positive feedback.

```
❯ grep lint-spell Makefile
	@echo " - lint-spell                       lint spelling"
	@echo " - lint-spell-fix                   lint spelling and fix issues"
lint: lint-frontend lint-backend lint-spell
lint-fix: lint-frontend-fix lint-backend-fix lint-spell-fix
.PHONY: lint-spell
lint-spell: lint-codespell
.PHONY: lint-spell-fix
lint-spell-fix: lint-codespell-fix
❯ git grep lint- -- .forgejo/
.forgejo/workflows/testing.yml:      - run: make --always-make -j$(nproc) lint-backend checks-backend # ensure the "go-licenses" make target runs
.forgejo/workflows/testing.yml:      - run: make lint-frontend
```
so how would you like me to invoke `lint-codespell` on CI? (without that would be IMHO very suboptimal and let typos sneak in)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/3270
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Yaroslav Halchenko <debian@onerussian.com>
Co-committed-by: Yaroslav Halchenko <debian@onerussian.com>

modules/markup/markdown/callout/github_legacy.go

@@ -51,7 +51,7 @@ func (g *GitHubLegacyCalloutTransformer) Transform(node *ast.Document, reader te
 			// color the blockquote
 			v.SetAttributeString("class", []byte("attention-header attention-"+calloutType))
 
-			// Create new parargaph.
+			// Create new paragraph.
 			attentionParagraph := ast.NewParagraph()
 			attentionParagraph.SetAttributeString("class", []byte("attention-title"))