docs: add support for custom BootstrapVue documentation link

What does this MR do?

Follow-up to !2388 (comment 685581410)

You can specify a bootstrapComponent parameter to add a link to the BootstrapVue documentation page. This works for BootstrapVue components that have a dedicated page, but doesn't currently work for components that don't have a dedicated page. For example BTableLite is documented in the BTable page: https://bootstrap-vue.org/docs/components/table#light-weight-tables

This MR adds support for the bootstrapComponentLink parameter that allows you to specify a custom link.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.