Add Sidecar Custom Types Bundle Instructions

Adds instructions for generating and using custom types to the Substrate API Sidecar page.

[x] I have added a label to this PR 🏷️
[x] I have run my changes through Grammarly
[ ] If this requires translations for the moonbeam-docs-cn repo, I have created a ticket for the translations in Jira
[ ] If pages have been moved around, I have created an additional PR in moonbeam-mkdocs to update redirects
[ ] If pages have been moved around, I have run the move-pages.py script to move the pages and update the image paths on the chinese repo
- [ ] After the script has been run, I have created an additional PR in moonbeam-docs-cn
[ ] If images have been added, I have run the compress-images.py script to compress the images.
[ ] If variables (in variables.yml) need to be updated (such as a name change), I have updated the moonbeam-docs-cn repo to use the new variables
[ ] If this page requires a disclaimer, I have added one

Please link to any corresponding PRs here.

[ ] Will need to create PR in moonbeam-docs repo to remove images
[ ] Will need to create PR in moonbeam-docs repo to remove variables
[ ] Will need to create PR in moonbeam-mkdocs repo to add redirects for Chinese site
[ ] No additional PRs are required after the translations are done

Please list any of the items that will need to be added or deleted after the translations are done here.

moonbeam-foundation / moonbeam-docs