dbt Core v1.8 docs beta init (#4727)

## What are you changing in this pull request and why?

Initializing dbt core v1.8 beta in the docs

We are technically in the `alpha` stage but we only have the `beta`
designation for the dropdown. The text clarifies it's an alpha where
appropriate.

## Checklist

- [x] Add versioning components, as described in [Versioning
Docs](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-entire-pages)
- [x] Add a note to the prerelease version [Migration
Guide](https://github.com/dbt-labs/docs.getdbt.com/tree/current/website/docs/docs/dbt-versions/core-upgrade)

- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.
- [x] For [docs
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#about-versioning),
review how to [version a whole
page](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
and [version a block of
content](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-blocks-of-content).
- [ ] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

Adding or removing pages (delete if not applicable):
- [x] Add/remove page in `website/sidebars.js`
- [x] Provide a unique filename for new pages
- [x] Add an entry for deleted pages in `website/static/_redirects`
- [x] Run link testing locally with `npm run build` to update the links
that point to deleted pages

website/dbt-versions.js

@@ -1,4 +1,8 @@
 exports.versions = [
+  {
+    version: "1.8",
+    isPrerelease: "true",
+  },
   {
     version: "1.7",
     EOLDate: "2024-10-30",

website/docs/docs/dbt-versions/core-upgrade/01-upgrading-to-v1.8.md

@@ -0,0 +1,25 @@
+---
+title: "Upgrading to v1.8 (alpha)"
+id: upgrading-to-v1.8
+description: New features and changes in dbt Core v1.8
+displayed_sidebar: "docs"
+---
+
+## Resources
+
+- Changelog (coming soon)
+- [CLI Installation guide](/docs/core/installation-overview)
+- [Cloud upgrade guide](/docs/dbt-versions/upgrade-core-in-cloud)
+- Release schedule (coming soon)
+
+## What to know before upgrading
+
+dbt Labs is committed to providing backward compatibility for all versions 1.x, with the exception of any changes explicitly mentioned below. If you encounter an error upon upgrading, please let us know by [opening an issue](https://github.com/dbt-labs/dbt-core/issues/new).
+
+## New and changed features and functionality
+
+Coming soon!
+
+## Quick hits
+
+Coming soon!