feat(markdown docs): add aliases to options and subcommands

[willemneal]

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Move cargo md-gen to an ignored test. This allows moving clap-markdown to dev-dependency allowing the use of a fork and removing an unneeded dependency since the markdown generation is pre-release.

[willemneal]

@leighmcculloch Is it reasonable to make an exception for cargo-deny it is a dev dependency under our control and only impacts generating docs. I'm not sure how long it will take my upstream changes to land.

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[elizabethengelman]

clap-markdown merged in Willem's PR! So, we can now use the upstream dep instead of our fork. One thing to note is that in a follow up PR on clap-markdown, there was a small change to where the aliases are included in the generated markdown:

• Print command aliases only in their dedicated section, not in the subcommands listing of the parent command.
• Move the command aliases out of the command header (simplifying the generated anchor link text) and into a bolded section underneath.

For example, using Willem's fork, the docs for would look like this for stellar keys public-key:

## `stellar keys`

Create and manage identities including keys and addresses

**Usage:** `stellar keys <COMMAND>`

###### **Subcommands:**

* `add` — Add a new identity (keypair, ledger, OS specific secure store)
* `public-key` **[alias: `address`]** — Given an identity return its address (public key)
...
## `stellar keys public-key` **[alias: `address`]**

Given an identity return its address (public key)

**Usage:** `stellar keys public-key [OPTIONS] <NAME>`

But with upstream clap-markdown, it removes the [alias: ...] and adds a Command Alias section at the bottom of each command section.

## `stellar keys`

Create and manage identities including keys and addresses

**Usage:** `stellar keys <COMMAND>`

###### **Subcommands:**

* `add` — Add a new identity (keypair, ledger, OS specific secure store)
* `public-key` — Given an identity return its address (public key)
...

## `stellar keys public-key`

Given an identity return its address (public key)

**Usage:** `stellar keys public-key [OPTIONS] <NAME>`

**Command Alias:** `address`

[leighmcculloch]

👍🏻 I think it's worth using upstream even with the difference, just so we continue to track upstream. Thanks @willemneal for contributing that upstream!