docs: Main docs update (#1304)

* Remove template in favor of official samples

* Fixed a variable name copy pasta mistake

line 35 was _database.GetData() instead of DBService.GetData()

* Experimental theme change

* Change paragraph, code, heading fonts
* Widen viewport

* Update DocFX.Plugins.LastModified v1.2.3

* Exclude Discord.API in docs

* Add remarks for SocketReaction properties

* Add examples for BaseSocketClient.Events

* Add additional clarification for some methods

* Move IUser and IGuildChannel examples

* Clarify several guides samples with notes

- Reword TypeReader comment to avoid giving the idea that the sample itself is "obsolete"
- Remove CommandException logging comment regarding C#7.0 as the version is now the standard across VS2017 and up
- Remove suggestion about handling result in command handler since it is now advised to use CommandExecuted instead
+ Add additional comment to clarify ctor for DI setup

* Add/migrate code examples

* Incorporate material design theme

License @ https://github.com/ovasquez

* Update installation and nightly guide

* Fix improper indentations made obvious by the widen viewport
* Fix minor grammar issues
+ Add installation for nightly build using dotnet CLI

* Fix nav level indentation

* Revise "Your First Bot" article

* Merge some paragraphs to avoid clutter while keeping readability
* Reword the use of command framework
+ Add additional warning/note about environment variable

* Add additional indent level

* Fix indentation text warping

* Remove connections sample

* Update logging sample

Remove redundant part of the sample

* Remove mention of RPC

* Remove misleading section about commands

- Remove command sample from complete snippet
* Revise "Your First Bot" command paragraphs
* Change wording to hint devs that additional command parser packages may be available, as more and more begin to crop up

* Update themes

* Add XML docs contribution guidelines


Update guidelines

* Update CommandExecuted remarks

* Fix precondition remarks typo
no one saw that ok

* Fix permission sample in docfx

* Fix IMessageChannel samples

* Update docs/_template/light-dark-theme/styles/docfx.vendor.minify.css

Co-Authored-By: Still34 <341464@gmail.com>

* Update docs/_template/light-dark-theme/styles/material.css

Co-Authored-By: Still34 <341464@gmail.com>

* Update docs/_template/light-dark-theme/styles/material.css

Co-Authored-By: Still34 <341464@gmail.com>

src/Discord.Net.Core/IDiscordClient.cs

@@ -63,7 +63,7 @@ namespace Discord
         ///     Gets a generic channel.
         /// </summary>
         /// <example>
-        ///     <code lang="cs" title="Example method">
+        ///     <code language="cs" title="Example method">
         ///     var channel = await _client.GetChannelAsync(381889909113225237);
         ///     if (channel != null &amp;&amp; channel is IMessageChannel msgChannel)
         ///     {
@@ -194,7 +194,7 @@ namespace Discord
         ///     Gets a user.
         /// </summary>
         /// <example>
-        ///     <code lang="cs" title="Example method">
+        ///     <code language="cs" title="Example method">
         ///     var user = await _client.GetUserAsync(168693960628371456);
         ///     if (user != null)
         ///         Console.WriteLine($"{user} is created at {user.CreatedAt}.";
@@ -212,7 +212,7 @@ namespace Discord
         ///     Gets a user.
         /// </summary>
         /// <example>
-        ///     <code lang="cs" title="Example method">
+        ///     <code language="cs" title="Example method">
         ///    var user = await _client.GetUserAsync("Still", "2876");
         ///    if (user != null)
         ///        Console.WriteLine($"{user} is created at {user.CreatedAt}.";
@@ -232,7 +232,7 @@ namespace Discord
         /// </summary>
         /// <example>
         ///     The following example gets the most optimal voice region from the collection.
-        ///     <code lang="cs">
+        ///     <code language="cs">
         ///         var regions = await client.GetVoiceRegionsAsync();
         ///         var optimalRegion = regions.FirstOrDefault(x =&gt; x.IsOptimal);
         ///     </code>