cmd/docgen: add HTTP extractor

[maddyblue]

Add a way to extract docs from the status.proto HTTP endpoint. These
can be imported into the docs project as needed.

Release note: None

[cockroach-teamcity]

This change is 

[maddyblue]

@taroface there's 2 markdown files here. Please comment on them and I can adjust. The current format is bad and I think you'll have a better idea of what you want.

[taroface]

@mjibson Thanks for generating these. I went ahead and mocked up some Markdown templates, because it seemed easier. I'm open to your feedback on any of this:

full.md

• I like the utility of the endpoints table at the top, but it may be more efficient to make each endpoint an h2 and have them appear in the article nav.
• Is there a way to scrape the error codes for each endpoint? I made up a placeholder for this, just in case.

{{site.data.alerts.callout_success}}
This page describes...
{{site.data.alerts.end}}

## Certificates

`GET /_status/certificates/{node_id}`

Brief description of this endpoint.

<a name="cockroach.server.serverpb.CertificatesRequest"></a>
### Request Parameters

| Field | Type | Label | Description |
| ----- | ---- | ----- | ----------- |
| node_id | [string](#string) |  | node_id is a string so that "local" can be used to specify that no forwarding is necessary. |

<a name="cockroach.server.serverpb.CertificatesResponse"></a>
### Response Parameters

| Field | Type | Label | Description |
| ----- | ---- | ----- | ----------- |
| certificates | [CertificateDetails](#cockroach.server.serverpb.CertificateDetails) | repeated |  |

<a name="cockroach.server.serverpb.CertificatesError"></a>
### Error Responses

| Error | Code | Description |
| ----- | ---- | ----------- |
...

## Details

`GET /_status/details/{node_id}`

...

hotranges.md

• Assuming this is one of the endpoint pages that will be mostly manual:

---
title: HotRanges
summary: The HotRanges endpoint returns hot ranges.
toc: true
---

Description of this endpoint.

### Method

`GET /_status/hotranges`

### Example Request

...

### Request Parameters

<div>
  {% include {{ page.version.version }}/path-to-hotrangesrequest %}
</div>

### Example Response

...

### Response Parameters

<div>
  {% include {{ page.version.version }}/path-to-hotrangesresponse %}
</div>

### Error Responses

<div>
  {% include {{ page.version.version }}/path-to-hotrangeserror %}
</div>

[maddyblue]

Ok I think the full markdown will work like that. How about for the single pages one, instead of including multiple separate generated pages, it just includes a single page that has all of the request/response/error data in it? I also don't think there's a way to do error responses. Those I think happen at a higher level, like there's one generic error response or something? It's not in the proto in any case.

[taroface]

For e.g. hotranges.md, the reason I'd want to have separate includes is to have the request/response parameter tables come after the example request or response. Is it much harder or messier to do it this way?

[maddyblue]

RFAL. I combined the response and request sections into one for the single page template so you should be able to do it in one include. Error responses are all the same:

cockroach/pkg/server/serverpb/status.proto

Line 293 in 8997ae2

message ResponseError {

[taroface]

@mjibson Looking at #50126, are there any per-endpoint authorization parameters we can also include in the markdown files?

[maddyblue]

Nope, auth is done in go files, not listed in protos.

[maddyblue]

RFAL. Any more comments on the markdown?

[taroface]

lgtm. thank you!

[maddyblue]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)