Docs: plugin migration guide 7.0.0 to 8.0.0 (#38911) (#39246)

* restructuring to new storybook inspired structure.

* added instructions on how to handle breaking changes in the legend.

* start adding migration steps for 7 -> 8

* added information about the data frame format changes.

* added plugin.json changes.

* added emotion 11 imports.

* added information about theme v1 being deprecated.

* added steps for backend plugin migration.

* added reference to the migrate hooks v6 -> v7.

* docs(pluginmigration): introduce getColorForTheme changes

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: Levente Balogh <balogh.levente.hu@gmail.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: Jack Westbrook <jack.westbrook@gmail.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: Levente Balogh <balogh.levente.hu@gmail.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: Levente Balogh <balogh.levente.hu@gmail.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: Levente Balogh <balogh.levente.hu@gmail.com>

* docs(migration-guide): introduce useStyles to v8 deprecation notes

* Apply suggestions from code review

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* suggestion from review.

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* docs(Migration Guide): update the example for `grafanaDependency`

* docs(Migration Guide): remove the time-series migration part for now

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: Ursula Kallio <73951760+osg-grafana@users.noreply.github.com>

* docs(Migration Guide): update wording about plugin signing

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/developers/plugins/migration-guide.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

Co-authored-by: Jack Westbrook <jack.westbrook@gmail.com>
Co-authored-by: Levente Balogh <balogh.levente.hu@gmail.com>
Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>
Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>
Co-authored-by: Ursula Kallio <73951760+osg-grafana@users.noreply.github.com>
(cherry picked from commit aba8af1)

Co-authored-by: Marcus Andersson <marcus.andersson@grafana.com>