Don't use @latest tag in CDN as it now points to v3

[gcleaves]

Editing docs so that CDN URLs don't point to @latest version since that tag now seems to be pointing to v3 of Vue.

Describe the PR

Update to Getting Started docs

PR checklist

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix (fixes a boo-boo in the code) - fix(...), requires a patch version update
• [ ] Feature (adds a new feature to BootstrapVue) - feat(...), requires a minor version update
• [ ] Enhancement (augments an existing feature) - feat(...), requires a minor version update
• [ ] ARIA accessibility (fixes or improves ARIA accessibility) - fix(...), requires a patch or minor version update
• [x] Documentation update (improves documentation or typo fixes) - chore(docs), requires a patch version update
• [ ] Other (please describe)

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• [ ] Yes (please describe since breaking changes require a minor version update)

The PR fulfills these requirements:

• [x] It's submitted to the dev branch, not the master branch
• [ ] When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
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• [ ] The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, chore(docs): fix typo in README, etc.). This is very important, as the CHANGELOG is generated from these messages, and determines the next version type (patch or minor).

If new features/enhancement/fixes are added or changed:

• [ ] Includes documentation updates
• [ ] Includes component package.json meta section updates (prop, slot and event changes/updates)
• [ ] Includes any needed TypeScript declaration file updates
• [ ] New/updated tests are included and passing (required for new features and enhancements)
• [ ] Existing test suites are passing
• [ ] CodeCov for patch has met target (all changes/updates have been tested)
• [ ] The changes have not impacted the functionality of other components or directives
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• [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

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[Hiws]

Thanks for the PR.

Though it's being addressed in this PR, along with a few additional places: https://github.com/bootstrap-vue/bootstrap-vue/pull/6623/ (if it ever gets merged)

[VividLemon]

https://github.com/bootstrap-vue/bootstrap-vue/pull/6623