Emit collapsed tables on `@dotnet-bot help` (#561)
Emit collapsed tables on `@dotnet-bot help` (#561)

Currently, the help message displays a wall of content in tabular form.
This change transforms:

> The following is a list of valid commands on this PR.  To invoke a command, comment the indicated phrase on
the PR
> **The following commands are valid for all PRs and repositories.**
>
> Comment Phrase | Action
>-------------- | ------
>@dotnet-bot test this please | Re-run all legs.  Use sparingly
>@dotnet-bot test ci please | Generates (but does not run) jobs based on changes to the groovy job definitions
in this branch
> @dotnet-bot help | Print this help message

to:

> The following is a list of valid commands on this PR.  To invoke a command, comment the indicated phrase on
the PR
>**The following commands are valid for all PRs and repositories.**
>
><details>
>  <summary>Click to expand</summary>
>
>Comment Phrase | Action
>-------------- | ------
>@dotnet-bot test this please | Re-run all legs.  Use sparingly
>@dotnet-bot test ci please | Generates (but does not run) jobs based on changes to the groovy job definitions
in this branch
>@dotnet-bot help | Print this help message
></details>

Note that this works in all non-Edge / non-IE browsers.

Refs:
* https://developer.microsoft.com/en-us/microsoft-edge/platform/status/detailssummary
* MicrosoftEdge/Status#480
tannergooding/dotnet-ci,tannergooding/dotnet-ci,dotnet/dotnet-ci,dotnet/dotnet-ci,mmitche/dotnet-ci,mmitche/dotnet-ci