Update readme (#3)

docs: Revamp README and add APK naming convention

This commit introduces a complete overhaul of the `README.md` to improve project presentation. Key changes include:
*   Adds a new header with the app icon and a Google Play badge.
*   Updates the feature graphic and includes a note distinguishing the OSS version from the Play Store version.
*   Refines the "Open Source Libraries" section to be more structured.
*   Moves the main preview image into the `docs` directory.

Additionally, the `app/build.gradle.kts` file is updated to configure a standardized naming convention for output APK files, including flavor, version, and build type.

* refactor copyright headers and fix README icon alignment

This commit introduces two main changes across the codebase:

The `spotless/copyright.kt` template have been updated for consistency:
*   The author name is simplified from "Episteme Authors" to "Episteme".
*   The contact email prefix is shortened from "Electronic mail:" to "mail:".

Second, the `README.md` file is adjusted to correct the vertical alignment of the app icon with the project title.

app/src/main/java/com/aryan/reader/epubreader/EpubReaderScreen.kt

@@ -1,6 +1,6 @@
 /*
  * Episteme Reader - A native Android document reader.
- * Copyright (C) 2026 Episteme Authors
+ * Copyright (C) 2026 Episteme
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as
@@ -15,7 +15,7 @@
  * You should have received a copy of the GNU Affero General Public License
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  *
- * Electronic mail: epistemereader@gmail.com
+ * mail: epistemereader@gmail.com
  */
 @file:OptIn(ExperimentalSerializationApi::class) @file:Suppress("VariableNeverRead")