docs: expand downporting page with article content

Incorporate the LinkedIn article 'Running abap2UI5 on older R/3
Releases': installation steps for old releases, cloud connectivity via
the RFC/HTTP connectors, and the four design principles that keep the
codebase compatible from ABAP 7.02 up to ABAP Cloud (basic technology
only, few SAP dependencies, version-independent code, generated 702
syntax).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J3CfpdPhr5mMXJGfmhCaLf

Author: claude

docs/advanced/downporting.md

@@ -14,6 +14,15 @@ To install on an older system, use the `702` branch:
 
 Some sample projects and other repositories also ship a downported version. Check whether a `702` branch is available.
 
+#### Installation
+The installation on an old release follows the normal [installation](/configuration/installation) flow with two adjustments:
+1. Pull the `702` branch with abapGit — abapGit itself is 7.02-compatible, so deployment to old ECC machines works the same way as everywhere else
+2. Create the HTTP handler (SICF service) with the 7.02-compatible handler syntax instead of the modern snippet
+
+After that, call your HTTP handler from the browser and abap2UI5 is ready for use — even on your old machine.
+
+If you later want to reach these apps from the cloud, use the [RFC Connector](/advanced/rfc): it calls abap2UI5 apps on legacy systems via BTP destination and RFC, integrating them into Steampunk, S/4 Public Cloud, Fiori Launchpads, or SAP Mobile Start. The [HTTP Connector](/advanced/http) offers the same over HTTP.
+
 #### How It Works
 The `702` branch is not maintained by hand — it is **generated** from `main` by an automated GitHub Actions workflow on every change. The pipeline runs [abaplint](https://abaplint.org)'s downport rule (`abaplint --fix` with a 7.02 target configuration), which rewrites modern syntax into 7.02-compatible equivalents, for example:
 
@@ -22,9 +31,22 @@ The `702` branch is not maintained by hand — it is **generated** from `main` b
 - `xsdbool( )` → `boolc( )`
 - string templates → concatenation where needed
 
-A few small compatibility fix-ups follow (e.g. replacing exception types that don't exist on old releases), and the result is committed to the `702` branch. Because the transformation is fully automatic, the downported version stays feature-identical with `main` — you never wait for a manual backport.
+A few small compatibility fix-ups follow (e.g. replacing exception types that don't exist on old releases), and the result is committed to the `702` branch. Because the transformation is fully automatic, the downported version stays feature-identical with `main` — you never wait for a manual backport. And since the main code line stays on 7.50 syntax, contributors write and debug modern ABAP instead of 7.02 syntax.
 
 The same mechanism runs in this project's CI (`npm run auto_downport`) to guarantee every change on `main` stays downportable.
 
+#### Why abap2UI5 Is Downportable
+Automatic syntax rewriting is only the last step. The codebase is compatible with everything from ABAP 7.02 up to the newest ABAP Cloud because of four design principles:
+
+**1. Only basic technology.** No OData services, CDS views, or new EML syntax — none of these exist on older ECCs. Conversely, no SAP GUI or other statements forbidden in ABAP Cloud. The only universally available technology is a plain HTTP service (defined via SICF on-premise, via Eclipse in the cloud), and that is all abap2UI5 needs for communication.
+
+**2. Few SAP dependencies.** SAP APIs like `/UI2/CL_JSON` or function modules may be missing or behave differently on certain releases, and their availability per release is poorly documented. abap2UI5 therefore replaces SAP dependencies with open-source projects where possible — most importantly [ajson](/technical/tools/ajson) for JSON handling, which is compatible from 7.02 to ABAP Cloud and ships bundled under the `z2ui5` namespace. Browse [dotabap.org](https://dotabap.org) for more open-source projects that can reduce your own dependencies.
+
+**3. Version-independent code.** Where a dependency is unavoidable and SAP offers different APIs per language version (e.g., generating UUIDs), abap2UI5 uses a hybrid approach: dynamic ABAP calls the new API in ABAP for Cloud and falls back to the classic API in Standard ABAP — one code line, no syntax errors on either side. Not always elegant, but very pragmatic.
+
+**4. Downported syntax.** The main code line is written in 7.50 syntax and the `702` branch is generated automatically, as described above.
+
+Keep these principles in mind if you want to set up your next abapGit project with compatibility for older releases.
+
 #### Further Reading
 Background article: [Running abap2UI5 on older R/3 Releases](https://www.linkedin.com/pulse/running-abap2ui5-older-r3-releases-downport-compatibility-abaplint-mjkle).