Migrate pg_bigm hosting from OSDN to GitHub. (#19)

Due to frequent access issues with OSDN, the pg_bigm Development Group
decided to officially move hosting from OSDN to GitHub. The previously
prepared GitHub mirror is now the official site.

This commit updates the documentation to fit GitHub style, converting formats
from HTML to Markdown, removing release notes (now on the releases page),
and fixing URLs to point to GitHub instead of OSDN.

Back-patch to pg_bigm 1.0.

README.md

@@ -1,25 +1,30 @@
 # pg_bigm
+
 The pg_bigm module provides full text search capability in [PostgreSQL](https://www.postgresql.org/).
 This module allows a user to create **2-gram** (bigram) index for faster full text search.
 
 pg_bigm is released under the [PostgreSQL License](https://opensource.org/licenses/postgresql), a liberal Open Source license, similar to the BSD or MIT licenses.
 
 ## Test Status
+
 [![Build Status](https://github.com/pgbigm/pg_bigm/actions/workflows/test.yml/badge.svg)](https://github.com/pgbigm/pg_bigm/actions/workflows/test.yml)
 
 ## Documentation
+
 ### English
-* [Release 1.2](https://pgbigm.github.io/pg_bigm/pg_bigm_en-1-2.html)
-* [Release 1.1](https://pgbigm.github.io/pg_bigm/pg_bigm_en-1-1.html)
-* [Release 1.0](https://pgbigm.github.io/pg_bigm/pg_bigm_en.html)
+
+* [Release 1.2](docs/pg_bigm_en.md)
+* [Release 1.1](../REL1_1_STABLE/docs/pg_bigm_en.md)
+* [Release 1.0](../REL1_0_STABLE/docs/pg_bigm_en.md)
 
 ### 日本語
-* [リリース1.2](https://pgbigm.github.io/pg_bigm/pg_bigm-1-2.html)
-* [リリース1.1](https://pgbigm.github.io/pg_bigm/pg_bigm-1-1.html)
-* [リリース1.0](https://pgbigm.github.io/pg_bigm/pg_bigm.html)
+
+* [リリース1.2](docs/pg_bigm.md)
+* [リリース1.1](../REL1_1_STABLE/docs/pg_bigm.md)
+* [リリース1.0](../REL1_0_STABLE/docs/pg_bigm.md)
 
 *****
 
 Copyright (c) 2017-2023, pg_bigm Development Group
 
-Copyright (c) 2012-2016, NTT DATA Corporation
+Copyright (c) 2012-2016, NTT DATA Corporation