Issue:
Bug: docs(analytics): Instructions to set up `currency_conversion` (third party dependency API)



## Requirements
After the changes by [this](#6418) PR, we need to update the documentation for `analytics` crate setup process.

## Reason
Due to the third party dependency on `currency_conversion` crate, we need to ensure the proper setup of this service as well.

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [x] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
Now that `analytics` depends on `currency_conversion` crate after this [PR](#6418), we need to include instructions in `analytics` documentation to set up this service.
## Fixes #6513 
### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
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Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
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This is related to the PR #6418 and documents the necessary prerequisite setup info.

## How did you test it?
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## Checklist
<!-- Put an `x` in the boxes that apply -->

- [ ] I formatted the code `cargo +nightly fmt --all`
- [ ] I addressed lints thrown by `cargo clippy`
- [ ] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Testing:

Patch:
diff --git a/crates/analytics/docs/README.md b/crates/analytics/docs/README.md
index 96218fc231c..eb5c26a6ba3 100644
--- a/crates/analytics/docs/README.md
+++ b/crates/analytics/docs/README.md
@@ -91,6 +91,44 @@ source = "kafka"
 
 After making this change, save the file and restart your application for the changes to take effect.
 
+## Setting up Forex APIs
+
+To use Forex services, you need to sign up and get your API keys from the following providers:
+
+1. Primary Service 
+   - Sign up for a free account and get your Primary API key [here](https://openexchangerates.org/).
+   - It will be in dashboard, labeled as `app_id`.
+
+2. Fallback Service
+   - Sign up for a free account and get your Fallback API key [here](https://apilayer.com/marketplace/exchangerate_host-api).
+   - It will be in dashboard, labeled as `access key`.
+
+### Configuring Forex APIs
+
+To configure the Forex APIs, update the `config/development.toml` or `config/docker_compose.toml` file with your API keys:
+
+```toml
+[forex_api]
+api_key = "YOUR API KEY HERE" # Replace the placeholder with your Primary API Key
+fallback_api_key = "YOUR API KEY HERE" # Replace the placeholder with your Fallback API Key 
+```
+### Important Note
+```bash
+ERROR router::services::api: error: {"error":{"type":"api","message":"Failed to fetch currency exchange rate","code":"HE_00"}}
+│
+├─▶ Failed to fetch currency exchange rate
+│
+╰─▶ Could not acquire the lock for cache entry
+```
+
+_If you get the above error after setting up, simply remove the `redis` key `"{forex_cache}_lock"` by running this in shell_
+
+```bash
+redis-cli del "{forex_cache}_lock"
+```
+
+After making these changes, save the file and restart your application for the changes to take effect.
+
 ## Enabling Data Features in Dashboard
 
 To check the data features in the dashboard, you need to enable them in the `config/dashboard.toml` configuration file.