[meta] Link checks & Redirects

### Related issues

* Allow redirects across repos
* **Problem to solve**: Sometimes we move content from one repo to another and want to redirect to the new page. We should be able to create redirects from a page in one repo to a page in another repo.
* **Issue:** [https://github.com/elastic/docs-builder/issues/847](https://github.com/elastic/docs-builder/issues/847)
* Force creation of redirects
* **Problem to solve**: We need to avoid our links dying after the site goes live (and in the runup to release). To avoid links dying, we should make people create redirects to the next best location when a page is moved or deleted.
* **Issue:** [https://github.com/elastic/docs-builder/issues/759](https://github.com/elastic/docs-builder/issues/759)
* Add error checking for removal of files/URLs by product
* **Problem to solve**: With the changes to the way we're handling versioning in the Docs V3, we'll have scenarios where URLs that are used in the product (for example, in error messages or logs or UIs) need to continue to exist (or have appropriate redirects) for as long as that product version is supported.
* **Issue:** [https://github.com/elastic/docs-builder/issues/736](https://github.com/elastic/docs-builder/issues/736)
* Add support for product link checking
* **Problem to solve**: We must ensure that links from Kibana (in the product) to documentation are valid
* **Issue:** [https://github.com/elastic/docs-builder/issues/123](https://github.com/elastic/docs-builder/issues/123),
* Link check to and from API docs
* https://github.com/elastic/docs-builder/issues/974
* A way to redirect from one page to another just for specific anchors
* **Problem to solve**: We need a way to redirect those specific anchors to the new subpage, without redirecting the entire parent page to the new subpage.
* **Issue:** [https://github.com/elastic/docs-builder/issues/670](https://github.com/elastic/docs-builder/issues/670)
* Force use of docs-builder links https://github.com/elastic/docs-builder/issues/1181

### RFC

In progress [here](https://docs.google.com/document/d/1Hyw-xGqT_s4Emw5acVLDKl9sfFh75ciicrgTMVWOQww/edit?tab=t.0).