[doc] Fix links in the documentation

docs/clients/mpd.md

@@ -5,7 +5,7 @@ You can - to some extent - use clients for MPD to control OwnTone.
 By default OwnTone listens on port 6600 for MPD clients. You can change
 this in the configuration file.
 
-Currently only a subset of the commands offered by MPD (see [MPD protocol documentation](http://www.musicpd.org/doc/protocol/)) are supported.
+Currently only a subset of the commands offered by MPD (see [MPD Protocol](https://mpd.readthedocs.io/en/latest/protocol.html)) are supported.
 
 Due to some differences between OwnTone and MPD not all commands will act the
 same way they would running MPD:
@@ -18,5 +18,5 @@ The following table shows what is working for a selection of MPD clients:
 
 |          Client                               |  Type  | Status          |
 | --------------------------------------------- | ------ | --------------- |
-| [mpc](http://www.musicpd.org/clients/mpc/)    | CLI    | Working commands: mpc, add, crop, current, del (ranges are not yet supported), play, next, prev (behaves like cdprev), pause, toggle, cdprev, seek, clear, outputs, enable, disable, playlist, ls, load, volume, repeat, random, single, search, find, list, update (initiates an init-rescan, the path argument is not supported)   |
+| [mpc](https://www.musicpd.org/clients/mpc/)    | CLI    | Working commands: mpc, add, crop, current, del (ranges are not yet supported), play, next, prev (behaves like cdprev), pause, toggle, cdprev, seek, clear, outputs, enable, disable, playlist, ls, load, volume, repeat, random, single, search, find, list, update (initiates an init-rescan, the path argument is not supported)   |
 | [ympd](http://www.ympd.org/)                  | Web    | Everything except "add stream" should work |

docs/integrations/spotify.md

@@ -44,7 +44,7 @@ The easiest way of accomplishing this may be with [Spocon](https://github.com/sp
 since it requires minimal configuration. After installing, create two pipes
 (with mkfifo) and set the configuration in the player section:
 
-```
+```conf
 # Audio output device (MIXER, PIPE, STDOUT)
 output = "PIPE"
 # Output raw (signed) PCM to this file (`player.output` must be PIPE)

docs/smart-playlists.md

@@ -1,4 +1,4 @@
-# OwnTone smart playlists
+# Smart Playlists
 
 To add a smart playlist to the server, create a new text file with a filename ending with .smartpl;
 the filename doesn't matter, only the .smartpl ending does. The file must be placed somewhere in your
@@ -153,7 +153,7 @@ This would match the last 10 music files added to the library.
 
 This generates a random set of, maximum of 10, rated Pop music tracks every time the playlist is queried.
 
-## Date operand syntax
+## Date Operand Syntax
 
 One example of a valid date is a date in yyyy-mm-dd format:
 
@@ -197,12 +197,11 @@ All dates, except for `YYYY-DD-HH`, are relative to the day of when the server e
 
 Note that `time_added after 4 weeks ago` and `time_added after last month` are subtly different; the former is exactly 4 weeks ago (from today) whereas the latter is the first day of the previous month.
 
-## Differences to mt-daapd smart playlists
+## Differences with MT-daapd Smart Playlists
 
-The syntax is really close to the mt-daapd smart playlist syntax (see
-http://sourceforge.net/p/mt-daapd/code/HEAD/tree/tags/release-0.2.4.2/contrib/mt-daapd.playlist).
+The syntax is really close to the mt-daapd smart playlist syntax (see [Multi-Threaded DAAP Daemon Code](https://sourceforge.net/p/mt-daapd/code/HEAD/tree/tags/release-0.2.4.2/contrib/mt-daapd.playlist).
 
-Even this documentation is based on the file linked above.
+Even this documentation is based on the document linked above.
 
 Some differences are: