doc: fix mkdocs build warnings

The README.md symlinks exist for GitHub browsing and collide with
index.md when mkdocs renders both; exclude them like TODO.md.

Two links pointed at anchors that never existed: features.md has bold
captions rather than headings, so "Automatic Reload" gets an explicit
attr_list anchor for the link from the front page, and the TTY link
now spells the actual heading, controlling-tty-for-services.

mkdocs build is silent after this.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

mkdocs.yml

@@ -12,6 +12,7 @@ extra_css:
 exclude_docs: |
   TODO.md
   ChangeLog.md
+  README.md
 
 nav:
   - Getting Started: