Improve the description format in the generated SDK PR

This is a follow-up task of this [feature request](https://github.com/Azure/azure-sdk-tools/issues/11385).

Currently, the description of the PR has pure text format. It would be better to render them in markdown format. It will need to update the `New-GitHubPullRequest` function in the [common powershell script](https://github.com/Azure/azure-sdk-tools/blob/e06e4924f38c8051ab55743d382ba7c1081ff82b/eng/common/scripts/Invoke-GitHubAPI.ps1#L148) to support JSON encoding appropriately.

[An example](https://github.com/Azure/azure-sdk-for-net/pull/51781#issue-3298100700) of the PR description looks like this:

Configurations: 'specification/azurefleet/AzureFleet.Management/tspconfig.yaml', API Version: 2022-12-01, SDK Release Type: beta, and CommitSHA: '4d0f681be6e2e629b714607e9968e7a85365342d' in SpecRepo: 'https://github.com/Azure/azure-rest-api-specs' Pipeline run: https://dev.azure.com/azure-sdk/internal/_build/results?buildId=5189076 Refer to https://eng.ms/docs/products/azure-developer-experience/develop/sdk-release/sdk-release-prerequisites to prepare for SDK release.