docs: add Portal Loop concept page (#1582)

## Description

This PR adds the Portal Loop concept page to the docs.
I propose we hold on merging this until `gno.land` is live.

EDIT: Portal Loop is live on [gno.land](https://gno.land) so we can
merge this PR.

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [x] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

docs/concepts/portal-loop.md

@@ -0,0 +1,60 @@
+---
+id: portal-loop
+---
+
+# Portal Loop
+
+Portal Loop is an always-up-to-date staging testnet that allows for using
+the latest version of Gno, Gno.land, and TM2. By utilizing the power of Docker
+& the [tx-archive](https://github.com/gnolang/tx-archive) tool, the Portal Loop can run the latest code from the 
+master branch on the [Gno monorepo](https://github.com/gnolang/gno), 
+while preserving most/all the previous the transaction data. 
+
+The Portal Loop allows for quick iteration on the latest version of Gno - without
+having to make a hard/soft fork. 
+
+Below is a diagram demonstrating how the Portal Loop works:
+```
+                    +----------------------------------+
+                    |       Portal Loop running        |  < ----+ 
+                    +----------------------------------+        |
+                                     |                          |
+                                     |                          |
+                                     v                          |
+                    +----------------------------------+        |
+                    |   Detect changes in 'master'     |        |
+                    +----------------------------------+        |
+                                     |                          |
+                                     |                          |
+                                     v                          |
+                    +----------------------------------+        |
+                    | Archive transaction data & state |        |    
+                    +----------------------------------+        |
+                                     |                          |
+                                     |                          |
+                                     v                          |
+                    +----------------------------------+        |
+                    |    Load changes from 'master'    |        |
+                    +----------------------------------+        |
+                                     |                          |
+                                     |                          |
+                                     v                          |
+                    +----------------------------------+        |
+                    |      Replay transaction data     |  ------+  
+                    +----------------------------------+
+```
+
+## Using the Portal Loop
+
+The Portal Loop deployment can be found at [gno.land](https://gno.land), while
+the exposed RPC endpoints can be found on `https://rpc.gno.land:443`. The RPC endpoint
+list can be found in the [reference section](../reference/rpc-endpoints.md).
+
+### A warning note
+
+While allowing for quick iteration on the most up-to-date software, the Portal Loop
+has some drawbacks:
+- If a breaking change happens on `master`, transactions that used the previous version of
+Gno will fail to be replayed, meaning **data will be lost**. 
+- Since transactions are archived and replayed during genesis, 
+block height & timestamp cannot be relied upon.

misc/docusaurus/sidebars.js

@@ -57,6 +57,7 @@ const sidebars = {
                 'concepts/effective-gno',
                 'concepts/proof-of-contribution',
                 'concepts/tendermint2',
+                'concepts/portal-loop',
                 'concepts/gno-modules',
                 'concepts/gno-test',
                 'concepts/from-go-to-gno',