Docs: plugin migration guide 7.0.0 to 8.0.0

[mckn]

What this PR does / why we need it:
This PR will restructure the migration guide to be structured more like the Storybook migration guide is structured.

It will also add migration steps for how to migrate a plugin from 7.x.x to 8.x.x

Which issue(s) this PR fixes:
Fixes #38911

Special notes for your reviewer:
If I have missed anything or describing anything in an incorrect way feel free to jump in and make changes.

[achatterjee-grafana]

Added more review.

[achatterjee-grafana]

Looks good. We can fix doc issues we may have missed in a future PR.

[grafanabot]

The backport to v8.0.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-38911-to-v8.0.x origin/v8.0.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x aba8af1d599ae4f67f9ffc92044eb477ca36455d
# Push it to GitHub
git push --set-upstream origin backport-38911-to-v8.0.x
git switch main
# Remove the local backport branch
git branch -D backport-38911-to-v8.0.x

Then, create a pull request where the base branch is v8.0.x and the compare/head branch is backport-38911-to-v8.0.x.

[grafanabot]

The backport to v8.1.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-38911-to-v8.1.x origin/v8.1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x aba8af1d599ae4f67f9ffc92044eb477ca36455d
# Push it to GitHub
git push --set-upstream origin backport-38911-to-v8.1.x
git switch main
# Remove the local backport branch
git branch -D backport-38911-to-v8.1.x

Then, create a pull request where the base branch is v8.1.x and the compare/head branch is backport-38911-to-v8.1.x.