refactor(exporter): mark extra pivot methods private

[damyanpetev]

Related to #14017 https://www.infragistics.com/products/ignite-ui-angular/docs/typescript/latest/classes/IgxExcelExporterService.html#addPivotRowHeaders is visible in the public API, when it shouldn't be. Guessing copied over from addPivotGridColumns which likely is in he same situation, so marking both private.

Additional information (check all that apply):

• [ ] Bug fix
• [ ] New functionality
• [x] Documentation
• [ ] Demos
• [ ] CI/CD

Checklist:

• [x] All relevant tags have been applied to this PR
• [ ] This PR includes unit tests covering all the new code (test guidelines)
• [ ] This PR includes API docs for newly added methods/properties (api docs guidelines)
• [ ] This PR includes feature/README.MD updates for the feature docs
• [ ] This PR includes general feature table updates in the root README.MD
• [ ] This PR includes CHANGELOG.MD updates for newly added functionality
• [ ] This PR contains breaking changes
• [ ] This PR includes ng update migrations for the breaking changes (migrations guidelines)
• [ ] This PR includes behavioral changes and the feature specification has been updated with them