Add version-stable anchors to the release notes template.

The current template embeds anchors from subheadings like "Compiler" that have
the same section subheading name; thus, the generated html ends up with `id`
attributes that have been uniquified (by adding a incremented suffix, e.g.
`#compiler-2`) but those `id` attributes will change (e.g. to `#compiler-3`) as
new releases come out.

The change in this PR enables one to write urls that end with e.g.
`#1.66.0-Compiler` and have that url remain usable (in terms of pointing to the
compiler subsection *for that release*), even in face of future releases.

FYI: in my local experimentation, it appears that in the context of hackmd,
visiting an anchor tag with no contents will end up scrolling to a point where
the hackmd toolbar obscures the subsection heading. This is unfortunate (though
still an improvement on the current state of affairs where one has *no option*
to link directly to these subsections in a version-stable manner). I am hoping
that there is not a similar problem for the RELEASES.md file when viewed on
github itself.

Author: pnkfelix

templates/relnotes.md

@@ -1,6 +1,8 @@
 Version {{version}} ({{date}})
 ==========================
 
+<a id="{{version}}-Language"></a>
+
 Language
 --------
 **relnotes**
@@ -9,6 +11,8 @@ Language
 **other**
 {{language_unsorted}}
 
+<a id="{{version}}-Compiler"></a>
+
 Compiler
 --------
 **relnotes**
@@ -17,6 +21,8 @@ Compiler
 **other**
 {{compiler_unsorted}}
 
+<a id="{{version}}-Libraries"></a>
+
 Libraries
 ---------
 **relnotes**
@@ -25,9 +31,13 @@ Libraries
 **other**
 {{libraries_unsorted}}
 
+<a id="{{version}}-Stabilized-APIs"></a>
+
 Stabilized APIs
 ---------------
 
+<a id="{{version}}-Cargo"></a>
+
 Cargo
 -----
 **relnotes**
@@ -36,9 +46,13 @@ Cargo
 **other**
 {{cargo_unsorted}}
 
+<a id="{{version}}-Misc"></a>
+
 Misc
 ----
 
+<a id="{{version}}-Compatibility-Notes"></a>
+
 Compatibility Notes
 -------------------
 **relnotes**
@@ -47,6 +61,8 @@ Compatibility Notes
 **other**
 {{compat_unsorted}}
 
+<a id="{{version}}-Internal-Changes"></a>
+
 Internal Changes
 ----------------