feat(gnoweb): "No render" page/component

[leohhhn]

Description

Defining a Render() function in realms is optional. Currently gnoweb presents an error if a realm that doesn't have a render func is requested. This should not be the case.

This PR also adds a VM error, RenderNotDeclared, which is to be returned when vm/qrender is called on a realm which does not have a Render() function declared.

I updated the status component to return the following in the aforementioned case:

Also adds another r/docs realm mentioning that a render function is optional in r/.

[Gno2D2]

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[codecov]

Codecov Report

Attention: Patch coverage is 89.28571% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gno.land/pkg/gnoweb/handler.go	60.00%	4 Missing ⚠️
gno.land/pkg/gnoweb/webclient_mock.go	93.75%	1 Missing ⚠️
gno.land/pkg/sdk/vm/errors.go	0.00%	1 Missing ⚠️

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[jefft0]

I tested this PR branch on my local machine. Indeed, if there is no Render function, gnoweb shows "This realm does not implement a Render() function." All tests pass (except "Merge requirements" as usual). Ready for a core dev to decide if this change is wanted.

[gfanton]

Can you add a test in handler_test.go please 🙏 , you can use something like TestWebHandler_NoRender

[leohhhn]

@gfanton

Added test in 18167fa & 1c92e8f