docs: fix broken RST markup (markdown links, :docs: role, changelog headings)

[0xbrayo]

Fixes markup that renders incorrectly or triggers Sphinx errors/warnings. No content changes.

• getting-started.rst: the macOS and Arch Linux autostart tabs used Markdown link syntax ([text](url)), which RST renders as literal brackets and parentheses. Converted to RST links. Also added the missing blank line after .. group-tab:: macOS.
• remote-server.rst, syncing.rst: :docs: isn't a valid role (ERROR: Unknown interpreted text role "docs"), so the links to the syncing and migrating pages didn't render. Changed to :doc:.
• changelog/v0_10.rst, changelog/v0_11.rst:
  • Four entries had half-converted Markdown links like (fixes[`#503 <…/aw-webui/issues/503>`__](…/activitywatch/issues/503)), which rendered with stray brackets and linked the wrong repo. They now link to the URL from the original Markdown.
  • Headings starting with an emoji (e.g. ✨ Features (26)) had underlines one character too short because docutils counts the emoji as double-width, causing "Title underline too short" warnings. Lengthened those underlines.

Built locally with Sphinx: the :docs: errors and emoji heading warnings are gone. The remaining "Duplicate explicit target name" warnings in v0_10.rst were already there and are left alone.