adds upgrade guide and apidocs

[somethings (lasomethingsomething)]

This moves upgrade-related files previously stored near the bottom of the left sidebar into a dedicated folder. It also consolidates API-related concepts information into a page that replaces one-paragraph pages and renames the folder "APIs" as we have more than one.

The pages in the guides > upgrades-migrations > admin folder will require some review at a later stage; for now I've just moved them.

[github-actions]

⚠️ Moved markdown files detected. Please ensure redirects are updated in .gitbook.yaml file.

[Micha Hobert (Isengo1989)]

Just quickly skimmed over it, and there is an issue with the files used in some MD files (rel paths are not working anymore)

The language 'script' is not loaded, falling back to 'txt' for syntax highlighting.
build error:
Could not resolve "./../../assets/concepts-api-storeApiLogic.svg" from "src/docs/concepts/framework/architecture/storefront-concept.md"
file: /home/runner/work/developer-portal/developer-portal/src/docs/concepts/framework/architecture/storefront-concept.md
Could not resolve "./../../assets/concepts-api-storeApiLogic.svg" from "src/docs/concepts/framework/architecture/storefront-concept.md"
file: /home/runner/work/developer-portal/developer-portal/src/docs/concepts/framework/architecture/storefront-concept.md
at getRollupError (file:///home/runner/work/developer-portal/developer-portal/node_modules/rollup/dist/es/shared/parseAst.js:401:41)
at error (file:///home/runner/work/developer-portal/developer-portal/node_modules/rollup/dist/es/shared/parseAst.js:397:42)
at ModuleLoader.handleInvalidResolvedId (file:///home/runner/work/developer-portal/developer-portal/node_modules/rollup/dist/es/shared/node-entry.js:21565:24)
at file:///home/runner/work/developer-portal/developer-portal/node_modules/rollup/dist/es/shared/node-entry.js:21525:26
 ELIFECYCLE  Command failed with exit code 1.
Error: Process completed with exit code 1.

[Su (sushmangupta)]

somethings (@lasomethingsomething) - Should we consider adding foundational Core concepts to the Architecture section https://developer.shopware.com/docs/concepts/framework/architecture/ ?

[somethings (lasomethingsomething)]

Su (@sushmangupta) Regarding arch concepts, go ahead or someone else can add -- no bandwidth for additional changes like that given the other dev changes forthcoming.

[Su (sushmangupta)]

Thanks for the PR somethings (@lasomethingsomething) and thanks for the support Jonas Elfering (@keulinho)