misc/genreadme,tempfork/pkgdoc,tsnet: generate README.md files from godocbradtest

Adds a CI check and Makefile target to keep opted-in directories'
README.md files in sync with their package godoc. For now tsnet (and
its sub-packages under tsnet/example) is the only opted-in tree. The
list of directories lives in misc/genreadme/genreadme.go as
defaultRoots, so CI and humans both just run `make genreadme` with no
arguments.

The genreadme workflow runs `make genreadme` and fails if any
README.md is out of date, pointing the user at the same command.

Along the way:

 - tempfork/pkgdoc now emits Markdown instead of plain text: headings
   become level-2 with no {#hdr-...} anchors, and [Symbol] doc links
   resolve to pkg.go.dev URLs, including for symbols in the current
   package (which the default Printer would otherwise emit as bare
   #Name fragments with no backing anchor in a README). Parsing no
   longer uses parser.ImportsOnly, so doc.Package knows the package's
   symbols and can resolve [Symbol] links at all.

 - genreadme also emits a pkg.go.dev Go Reference badge at the top of
   a library package's README; suppressed for package main.

 - tsnet/tsnet.go's package godoc is expanded in idiomatic godoc
   syntax — [Type], [Type.Method], reference-style [link]: URL
   definitions — rather than Markdown-flavored [text](url) or
   backtick-quoted identifiers, so that both pkg.go.dev and the
   generated README.md render cleanly from a single source.

Fixes #19431
Fixes #19483
Fixes #19470

Change-Id: Iade21dc87921ab2bbb65951e5fb373b597fed29e
Signed-off-by: Walter Poupore <walterp@tailscale.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/tsnet/example/tsnet-services/tsnet-services.go b/tsnet/example/tsnet-services/tsnet-services.go
index d72fd68fd..4604e8d3f 100644
--- a/tsnet/example/tsnet-services/tsnet-services.go
+++ b/tsnet/example/tsnet-services/tsnet-services.go
@@ -8,17 +8,16 @@
 //  1. Add access controls which (i) define a new ACL tag, (ii) allow the demo
 //     node to host the Service, and (iii) allow peers on the tailnet to reach
 //     the Service. A sample ACL policy is provided below.
-//
 //  2. [Generate an auth key] using the Tailscale admin panel. When doing so, add
 //     your new tag to your key (Service hosts must be tagged nodes).
-//
 //  3. [Define a Service]. For the purposes of this demo, it must be defined to
 //     listen on TCP port 443. Note that you only need to follow Step 1 in the
 //     linked document.
+//  4. Run the demo on the command line (step 4 command shown below).
 //
-//  4. Run the demo on the command line:
+// Command for step 4:
 //
-//     TS_AUTHKEY=<yourkey> go run tsnet-services.go -service <service-name>
+//	TS_AUTHKEY=<yourkey> go run tsnet-services.go -service <service-name>
 //
 // The following is a sample ACL policy for step 1:
 //