[Doc] Fix Modal/Portal reference docs

[jcalcaben]

Description

This PR fixes the reference doc generation for the Modal component, which has been renamed to Portal.

Related Issue

Closes PWA-641

Acceptance

any developer

Verification Stakeholders

any developer

Specification

Verification Steps

1. Navigate to the pwa-devdocs directory: cd pwa-devdocs
2. Build the preview site yarn develop
3. Verify build succeeds without any errors
4. Use the top nav to navigate to the Venia UI API section
5. Verify Portal topic is listed on the left nav and points to the correct topic
6. Verify Modal topic is not listed on the left nav

Screenshots / Screen Captures (if appropriate)

Checklist

• I have added tests to cover my changes, if necessary.

• I have updated the documentation accordingly, if necessary.

[PWAStudioBot]

	Messages
📖	Access a deployed version of this PR here. Make sure to wait for the "pwa-pull-request-deploy" job to complete.
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).
📖	Associated JIRA tickets: PWA-641.

Generated by 🚫 dangerJS against 7f261c1

[devops-pwa-codebuild]

Performance Test Results

The following fails have been reported by WebpageTest. These numbers indicates a possible performance issue with the PR which requires further manual testing to validate.

https://pr-2450.pwa-venia.com : LH Performance Expected 0.85 Actual 0.55, LH Best Practices Expected 1 Actual 0.92
https://pr-2450.pwa-venia.com/venia-tops.html : LH Performance Expected 0.75 Actual 0.33, LH Best Practices Expected 1 Actual 0.92
https://pr-2450.pwa-venia.com/valeria-two-layer-tank.html : LH Performance Expected 0.8 Actual 0.5, LH Accessibility Expected 0.9 Actual 0.89, LH Best Practices Expected 1 Actual 0.92

[jimbo]

@jcalcaben Thanks for catching this. 👍

[tjwiebell]

Re-approve after force push fun time 🎉

[dpatil-magento]

Verification steps look good.