docs: Documentation December Update (#1218)

* Fix broken link (#11)

* Fix typos and improve wording

* Add information for IGuildUser

+ Add GetPermission sample
+ Add ModifyAsync remarks

* Add information for IGuildChannel

+ Add ModifyAsync remarks
+ Add GetOverwritePermissionAsync examples

* Add warning for Direction.Around

* Fix indentations and references

* Move IRole.ModifyAsync sample

* Add information for IUser

+ Add example, remarks for Get(Default)AvatarUrl
+ Add example, remarks for GetOrCreateDMChannelAsync
+ Add missing remarks/summary/returns for other properties of the class

* Change verbs used in IVoiceState summary/remarks

* Add additional explanation for IGuildUser.RoleIds

* Change verbs used in IMessage summary/remarks

* Clarify IUserMessage Add/RemoveReactionAsync samples

* Fix command handler sample typo

* Add information for DiscordSocketConfig

+ Add remarks/example to the class
+ Add remarks to AlwaysDownloadUsers

* Fix documentation for SlowMode

* Add additional remarks for Guild/TextChannelProperties

* Update DocFx.Plugins.LastModified to v1.2.0
This should drastically improve docfx build time.

* Add missing dependencies

* Update DocFx.Plugins.LastModified to v1.2.1

Improve performance

* Update DocFx.Plugins.LastModified to v1.2.2

* Clarify deployment.md

+ Rewritten .NET Core deployment strategies for better clarification
    * Split deployment types into framework-dependent and self-contained
    * Clarify the benefits of using different types of publishing
    * Include a sample of how to execute dotnet application with the dotnet command in a TIP dialog for visibility

* Update post-execution article and samples

+ This change is to reflect changes made in https://github.com/RogueException/Discord.Net/pull/1164, where CommandInfo is now passed into the CommandExecuted event as an Optional<T>

* Update DocFX.Plugin.DescriptionGenerator to v1.1.1

* Adjust according to recent CommandExecuted changes

See:
+ f549da50e0
+ 6260749095

* Add further documentation for https://github.com/RogueException/Discord.Net/pull/1037

* Add partial documentation for the precondition helper class

* Include CHANGELOG.md in docs

* Revise post-execution docs
* Fix incorrect Optional<T> usage
* Indent some sample code and add a comment reminding the user that the post-execution basic sample code is not ideal.

* Streamline docs for Attachment
+ This commit also adds further explanation for why Embeds and Attachments are read-only collections

* Add further documentation for MessageActivity and MessageApplication

* Add caching-related docs to ISocketMessageChannel

* Add missing documentation inheritance for SyncPermissionsAsync

* Streamline documentation process

This is done by changing the documentation of the implementations required by interfaces to redirect to the interface method instead (e.g., SocketDMChannel#GetMessagesAsync refer to IMessageChannel.GetMessagesAsync within the remarks of the method).

* Cleanup 92bf8363ca

* Update src/Discord.Net.Core/Entities/Channels/Direction.cs

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

* Update src/Discord.Net.Core/Entities/Channels/Direction.cs

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

* Update src/Discord.Net.Core/Entities/Channels/GuildChannelProperties.cs

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

* Update src/Discord.Net.WebSocket/DiscordSocketConfig.cs

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

* Update according to PR suggestions

* Reword sentences of deployment article for clarification & remove mention of portability
* Fix typos/grammar errors within TextChannelProperties

* Add the logo SVG to the page navbar

* Implement changing logo image based on theme color using CSS background image

* Add a favicon

* use the purple logomark instead of white

* hack? set the title to navbar svg to read "Discord.Net Docs"

docs/marketing/logo/SVG/Logomark Purple.svg

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