[Doc] Initialize headless docs content based on RA docs

[slax57]

Problem

Now that the headless docs website is ready, we need to fill its content.

Solution

Create the headless docs website content by adapting the RA docs where applicable.

TODO

• Getting Started: to write

Guides & Concepts

• General Concepts
• Data Fetching
• CRUD Pages: replace by base components, remove layouts and filters
• Forms
• Security
• Routing
• I18N
• Preferences
• All Features (curated)

App Configuration

• <Admin> -> CoreAdmin
• <Resource>
• <CustomRoutes>

Data Fetching

• all

Security

• all

List Page

• Introduction (curated)
• Filtering the List (curated)
• <ListBase>
• <InfiniteListBase>
• <ListIterator>
• <FilterLiveForm>
• <WithListContext>
• useListContext
• useList
• useListController
• useUnselect
• useUnselectAll
• <DataTable> (will be done later)

Creation & Edition Pages

• Introduction
• Form Validation
• <CreateBase>
• <EditBase>
• <Form>
• useCreateContext
• useCreateController
• useEditContext
• useEditController
• useSaveContext
• useRecordFromLocation
• useRegisterMutationMiddleware
• useUnique

Show Page

• Introduction
• <ShowBase>
• useShowContext
• useShowController

Common

• <WithRecord>
• useRecordContext
• useGetRecordId
• useNotify
• useRedirect
• useRefresh

Fields

• Introduction
• Fields for Relationships
• <ArrayField> <- could be moved to ra-core
• <FieldTitle> (will be done later)
• <ReferenceFieldBase>
• <ReferenceArrayFieldBase>
• <ReferenceManyFieldBase>
• <ReferenceManyCountBase>
• <ReferenceOneFieldBase>
• useFieldValue

Inputs

• Introduction
• <ArrayInput> <- NO EQUIVALENT! Should provide an ArrayInputBase -- will be done later
• <InputHelperText> <- could be moved to ra-core -- will be done later
• <ReferenceInputBase> (will be done later)
• <ReferenceArrayInputBase> (will be done later)
• useInput

Preferences

• useStore
• useRemoveFromStore
• useResetStore
• useStoreContext

I18N Provider & Translations

• Setting Up
• Supported Locales
• Translating UI Components
• Writing an i18nProvider
• <Translate>
• useTranslate
• useLocaleState

Other Hooks & Components

• useCheckForApplicationUpdate (will be done later)
• <RecordRepresentation>
• useGetRecordRepresentation

Recipes

• Caching
• Unit Testing

---

• Confirm Enterprise features are out of scope for now
• Should I copy the Community page?
• Contributing page?
• Clean up img folder
• Fix videos
• Actually no need for manually using CoreAdminUI + CoreAdminContext, CoreAdmin does that!
• Global check: ReferenceInput can easily be replaced by ReferenceInputBase
• Improve props tables layout (1st column gets cut)
• Manually order sidebar entries
• Rename page title from 'React Admin Headless' to 'ra-core'

Screenshots

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• [ ] The PR includes unit tests (if not possible, describe why)
• [ ] The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

Also, please make sure to read the contributing guidelines.