Add v1.20 documentation (#28)

Modified `loop_docs.sh` to take `version` and `locale` and params so do not need other `loop_docs*` scripts.

Need confirmation with version information for 1.20 and latest.
Like version in `static/20-swagger.json` and `static/latest-swagger.json`, and version used to replace `{{< version >}}`

Will wait for a stable version for 1.20

![Screen Shot 2023-06-14 at 12.12.18](/attachments/1b316978-0daf-46bb-90a7-632a0fc80898)

Reviewed-on: https://gitea.com/gitea/gitea-docusaurus/pulls/28
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: HesterG <hestergong@gmail.com>
Co-committed-by: HesterG <hestergong@gmail.com>

docusaurus.config.js

@@ -55,12 +55,18 @@ const config = {
           },
           versions: {
             current: {
-              label: '1.20-dev', // path is kept as next for dev (so users can always find "nightly" docs)
+              label: 'next', // path is kept as next for dev (so users can always find "nightly" docs)
+              banner: 'unreleased',
             },
-            1.19 : {
-              label: '1.19.3'
+            '1.20': {
+              label: '1.20.0-rc0',
+              banner: 'none',
+            },
+            '1.19': {
+              label: '1.19.3',
             }
           },
+          lastVersion: '1.19',
           async sidebarItemsGenerator({defaultSidebarItemsGenerator, ...args}) {
             const {item} = args;
             // Use the provided data to generate a custom sidebar slice