doc/modules/gomod-ref: document the ignore directive

[abhay1999]

The go.mod reference page at https://go.dev/doc/modules/gomod-ref was
missing the ignore directive, even though it is documented in the Go
Modules Reference at https://go.dev/ref/mod#go-mod-file-ignore.

This adds a full ignore section following the same structure as the
existing exclude and retract sections, with syntax, examples (relative
path, named directory, and block form), and usage notes. The intro
paragraph is also updated to mention ignore alongside replace and
exclude.

Fixes golang/go#78460

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Abhay Chaurasiya:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/762520.
After addressing review feedback, remember to publish your drafts!