GitHub pages improvements - Element X Android Gallery

This is a list of things we can do later to improve the GitHub page of the project:

- [x] Update the URL (the fragment) when the form is updated (so also add a value for image width and a value for `Show all screenshots`)
- [x] add ids to sections (setting `id="AboutView"` to the table header), to be able to navigate quickly to a section, and add a way to copy the link quickly as well
- [x] Replace the `1` in the JS data to notify that a screenshot is available for a specific language with a day timestamp, so that the Web page can filter by date. Usage will be for the user to see only the new change since a specific date. Indeed it's hard to see what have changed, to check for new screenshot and regressions on existing ones.
- [x] add a date of build in the footer (https://github.com/element-hq/element-x-android/pull/7544)
- [ ] globally improve the Html / Javascript / Css code 🙈
- [ ] improve the form in the header
- [ ] Add a first welcome page?
- [ ] TBC...

Other:

- [x] Add links to the GitHub page in the doc (main README , i18n doc, design, etc.). Note that the page supports passing a list of languages in the URL fragment (so we may create a link per languages, such as https://element-hq.github.io/element-x-android/?l=uk for instance