bind avy-goto-line to `SPC y`

After the change in 1d340dcc77,
there was some grumbling in gitter that `SPC L` is not as convenient
as `SPC l`. Apparently avy-goto-line is a popular function.

`SPC y` is chosen because it is:

- currently unused (as far as I can tell).
- sort of mnemonic because "avy" has a "y" in it.
- doesn't require SHIFT to type
- is accessible even on non-US keyboards
  (there were suggestions such ";" and "\" that are not as accessible
   on non-US keyboards)

This change also updates the documentation regarding the change from
ace-jump-mode to avy.

doc/DOCUMENTATION.org

@@ -96,7 +96,7 @@
    - [[Navigating][Navigating]]
      - [[Point/Cursor][Point/Cursor]]
        - [[Smooth scrolling][Smooth scrolling]]
-     - [[Vim motions with ace-jump mode][Vim motions with ace-jump mode]]
+     - [[Vim motions with avy][Vim motions with avy]]
        - [[ace-link mode][ace-link mode]]
      - [[Window manipulation][Window manipulation]]
        - [[Window manipulation key bindings][Window manipulation key bindings]]
@@ -1432,23 +1432,23 @@ On Windows, you may want to disable it. To disable the smooth scrolling set the
 (setq-default dotspacemacs-smooth-scrolling t)
 #+END_SRC
 
-*** Vim motions with ace-jump mode
-Spacemacs uses the =evil= integration of [[https://github.com/winterTTr/ace-jump-mode][ace-jump mode]] which enables the
-invocation of =ace-jump-mode= during motions.
+*** Vim motions with avy
+Spacemacs uses the =evil= integration of [[https://github.com/abo-abo/avy][avy]] which enables the
+invocation of =avy= during motions.
 
 It is useful for deleting visually a set of lines, try the following sequence in
-a buffer containing some text: ~d SPC L~
+a buffer containing some text: ~d SPC y~
 
 | Key Binding | Description                                        |
 |-------------+----------------------------------------------------|
-| ~SPC SPC~   | initiate ace jump word mode                        |
-| ~SPC L~     | initiate ace jump line mode                        |
+| ~SPC SPC~   | initiate avy jump word                             |
+| ~SPC y~     | initiate avy jump line                             |
 | ~SPC `~     | go back to the previous location (before the jump) |
 
 *Hint*: you may change to char mode by ~C-c C-c~ in word mode.
 
 **** ace-link mode
-Similar to =ace-jump-mode=, [[https://github.com/abo-abo/ace-link][ace-link]] allows one to jump to any link in
+Similar to =avy=, [[https://github.com/abo-abo/ace-link][ace-link]] allows one to jump to any link in
 =help-mode= and =info-mode= with two key strokes.
 
 | Key Binding | Description                                           |
@@ -2389,7 +2389,7 @@ Comments are handled by [[https://github.com/redguardtoo/evil-nerd-commenter][ev
     | ~SPC c y~   | comment and yank          |
     | ~SPC c Y~   | invert comment and yank   |
 
-*Tips:* To comment efficiently a block of line use the combo ~SPC ; SPC L~
+*Tips:* To comment efficiently a block of line use the combo ~SPC ; SPC y~
 
 *** Deleting files
 Deletion is configured to send deleted files to system trash.

doc/FAQ.org

@@ -72,13 +72,13 @@ check out the =packages.el= file in the respective folders in the
 
 ** How can I change or define an alias for an =evil-leader= prefix?
 It is possible to change an =evil-leader= prefix by binding its keymap to
-another sequence. For instance, if you want to switch ~SPC L~
+another sequence. For instance, if you want to switch ~SPC y~
 (=evil-avy-goto-line=) with ~SPC l~ (=perspectives= layer) to make the later
 easier to reach, you can use:
 
 #+begin_src emacs-lisp
 (evil-leader/set-key
-  "L" (lookup-key evil-leader--default-map "l")
+  "y" (lookup-key evil-leader--default-map "l")
   "l" 'evil-avy-goto-line)
 #+end_src
 

layers/+distribution/spacemacs/packages.el

@@ -399,7 +399,7 @@
       (setq avy-background t)
       (evil-leader/set-key
         "SPC" 'avy-goto-word-or-subword-1
-        "L" 'avy-goto-line
+        "y" 'avy-goto-line
         "xo" 'spacemacs/avy-open-url))
     :config
     (progn

layers/+vim/evil-commentary/README.org

@@ -29,5 +29,5 @@ To use this contribution add it to your =~/.spacemacs=.
 | ~SPC ;~     | comment operator              |
 | ~gcc~       | comment current line          |
 | ~gcap~      | comment paragraphs            |
-| ~gc SPC L~  | comment up to a line with avy |
+| ~gc SPC y~  | comment up to a line with avy |
 | ~gy~        | comment and yank              |