Links for README.org + increase timeout for load_sync (nvim-orgmode#782)

* docs(README): add custom_ids and fix relative links

In nvim-orgmode, you can point to a headline either by using the star
syntax (* HEADLINE) or by using a custom-id in the properties drawer of
the headline, and then refer to it using the hash syntax (#custom-id).

This commit adds custom_ids for all the headlines referred to into the
README.org file and fixes relative links to other files in the
repository.

* fix(init): increment the default timeout to 15 seconds

On slow devices (for example Android smartphones using Termux), loading
a lot of org files takes more than 5 seconds, which will make the
default timeout for the promise to expire, leaving the plugin in a
semi-loaded state where some functionalities may not work, forcing the
user to reload the plugin multiple times hoping that the system has
something already cached and it will take less than 5 seconds.

README.org

@@ -10,7 +10,7 @@
 
 Orgmode clone written in Lua for Neovim 0.9.4+
 
-[[#setup][Setup]] • [[/DOCS.md][Docs]] • [[#showcase][Showcase]] • [[#treesitter-info][Treesitter]] • [[#troubleshoot][Troubleshoot]] • [[#plugins][Plugins]] • [[CONTRIBUTING.md][Contributing]] • [[#thanks-to][Kudos]]
+[[#setup][Setup]] • [[file:./DOCS.md][Docs]] • [[#showcase][Showcase]] • [[#treesitter-info][Treesitter]] • [[#troubleshoot][Troubleshoot]] • [[#plugins][Plugins]] • [[file:./CONTRIBUTING.md][Contributing]] • [[#thanks-to][Kudos]]
 
 #+HTML:</div>
 
@@ -79,6 +79,9 @@ call dein#add('nvim-orgmode/orgmode')
 #+HTML:</details>
 
 *** Setup
+:PROPERTIES:
+:CUSTOM_ID: setup
+:END:
 
 Note that this setup is not needed for [[https://github.com/folke/lazy.nvim][lazy.nvim]]
 since instructions above covers full setup
@@ -150,6 +153,9 @@ If you are new to Orgmode, see [[/DOCS.md#getting-started-with-orgmode][Getting
 or a hands-on [[https://github.com/nvim-orgmode/orgmode/wiki/Getting-Started][tutorial]] in our wiki.
 
 ** Showcase
+:PROPERTIES:
+:CUSTOM_ID: showcase
+:END:
 
 *** Agenda
 
@@ -176,6 +182,9 @@ or a hands-on [[https://github.com/nvim-orgmode/orgmode/wiki/Getting-Started][tu
 [[https://user-images.githubusercontent.com/1782860/123550227-e8605800-d76c-11eb-96f6-c0a677d562d4.gif]]
 
 ** Treesitter Info
+:PROPERTIES:
+:CUSTOM_ID: treesitter-info
+:END:
 
 The built-in treesitter parser is used for parsing the org files.
 
@@ -184,6 +193,9 @@ The built-in treesitter parser is used for parsing the org files.
 - LaTex is still highlighted through syntax file
 
 ** Troubleshoot
+:PROPERTIES:
+:CUSTOM_ID: troubleshoot
+:END:
 
 *** Indentation is not working
 
@@ -329,6 +341,9 @@ More info on issue [[https://github.com/nvim-orgmode/orgmode/issues/281#issuecom
 Link to detailed documentation: [[DOCS.md][DOCS]]
 
 ** Plugins
+:PROPERTIES:
+:CUSTOM_ID: plugins
+:END:
 
 - [[https://github.com/chipsenkbeil/org-roam.nvim][org-roam.nvim]] - Implementation of [[https://orgroam.com][Org-roam]] knowledge management system
 - [[https://github.com/nvim-orgmode/telescope-orgmode.nvim][telescope-orgmode.nvim]] - Telescope extension to find headlines, refile and insert links
@@ -381,6 +396,9 @@ Hosted documentation is on: [[https://nvim-orgmode.github.io/][https://nvim-orgm
 - :white_square_button: Support evaluating code blocks
 
 ** Thanks to
+:PROPERTIES:
+:CUSTOM_ID: thanks-to
+:END:
 
 - [[https://github.com/dhruvasagar][@dhruvasagar]] and his [[https://github.com/dhruvasagar/vim-dotoo][vim-dotoo]] plugin
   that got me started using orgmode. Without him this plugin would not happen.

lua/orgmode/init.lua

@@ -50,7 +50,7 @@ function Org:init()
     :new({
       paths = require('orgmode.config').org_agenda_files,
     })
-    :load_sync()
+    :load_sync(true, 20000)
   self.agenda = require('orgmode.agenda'):new({
     files = self.files,
   })