docs: update calendar and link releases notes for 20.1.8

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6748>
diff --git a/docs/release-calendar.rst b/docs/release-calendar.rst
index c61d091..f6cfb65 100644
--- a/docs/release-calendar.rst
+++ b/docs/release-calendar.rst
@@ -30,9 +30,7 @@
 +--------+---------------+------------+-----------------+-----------------------------------------+
 | Branch | Expected date | Release    | Release manager | Notes                                   |
 +========+===============+============+=================+=========================================+
-| 20.1   | 2020-09-16    | 20.1.8     | Eric Engestrom  |                                         |
-|        +---------------+------------+-----------------+-----------------------------------------+
-|        | 2020-09-30    | 20.1.9     | Eric Engestrom  | Last planned release of the 20.1 series |
+| 20.1   | 2020-09-30    | 20.1.9     | Eric Engestrom  | Last planned release of the 20.1 series |
 +--------+---------------+------------+-----------------+-----------------------------------------+
 | 20.2   | 2020-09-16    | 20.2.0-rc5 | Dylan Baker     | or 20.2.0 final                         |
 +--------+---------------+------------+-----------------+-----------------------------------------+
diff --git a/docs/relnotes.rst b/docs/relnotes.rst
index e623b52..5e2ddd1 100644
--- a/docs/relnotes.rst
+++ b/docs/relnotes.rst
@@ -3,6 +3,7 @@
 
 The release notes summarize what's new or changed in each Mesa release.
 
+-  :doc:`20.1.8 release notes <relnotes/20.1.8>`
 -  :doc:`20.1.7 release notes <relnotes/20.1.7>`
 -  :doc:`20.1.6 release notes <relnotes/20.1.6>`
 -  :doc:`20.1.5 release notes <relnotes/20.1.5>`
@@ -313,6 +314,7 @@
    :maxdepth: 1
    :hidden:
 
+   relnotes/20.1.8
    relnotes/20.1.7
    relnotes/20.1.6
    relnotes/20.1.5