feature: add edit button options for gh, gh-dev and cms

[bh0fer]

Adds siteConfig options:

export type ShowEditThisPage = 'always' | 'never' | 'loggedIn' | 'teachers' | 'admins';
export type EditThisPageOption = 'github' | 'github-dev' | 'cms';
/**
 * wheter to show the "Edit this page" links on docs and blog pages.
 */
showEditThisPage?: ShowEditThisPage;

/**
 * Options for which edit links to show. Only relevant if `showEditThisPage` is not 'never'.
 * @default ['github', 'github-dev', 'cms']
 * @example To only show the GitHub edit link:
 * ```ts
 * showEditThisPageOptions: ['github']
 * ```
 */
showEditThisPageOptions?: EditThisPageOption[];

/**
 * the URL to the CMS to edit the page. Defaults to '/cms/', but can be
 * redirected to a different path
 * @default '/cms/'
 * @example 'https://teaching-dev.gbsl.website/cms/'
 *
 * OrganizationName and ProjectName will be appended automatically.
 */
editThisPageCmsUrl?: string;

and introduces edit badges at the bottom of the page:

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[SilasBerger]

LGTM, cool addition, thanks 🥳